#############################################################################
##
#W  tietze.xml
#Y  Copyright (C) 2021                                   Tom Conti-Leslie
##                                                       Ben Spiers
##
##  Licensing information can be found in the README file of this package.
##
#############################################################################
##

<#GAPDoc Label="StzPresentation">
  <ManSection>
    <Oper Name="StzPresentation" Arg="S"/>
    <Returns>
      A semigroup Tietze (Stz) object.
    </Returns>
    <Description>
      If <A>s</A> is an fp semigroup (<Ref Filt="IsFpSemigroup"
      BookName="ref"/>), then this function returns a modifiable object
        representing the generators and relations of <A>s</A>.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");;
gap> AssignGeneratorVariables(F);;
gap> S := F / [[a * b, c], [b * c, a], [c * a, b]];
<fp semigroup with 3 generators and 3 relations of length 12>
gap> stz := StzPresentation(S);
<fp semigroup presentation with 3 generators and 3 relations
 with length 12>]]></Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="IsStzPresentation">
  <ManSection>
    <Filt Name="IsStzPresentation" Arg="stz"/>
    <Returns>
      <K>true</K> or <K>false</K>.
    </Returns>
    <Description>
      Every semigroup Tietze object is an element of the category
      <C>IsStzPresentation</C>. Internally, each Stz object contains a list of
      generators (each represented as a string) and a list of relations (each
      represented as a pair of <C>LetterRep</C> words, see <Ref
      Oper="LetterRepAssocWord" BookName="ref"/>).  These generator and
      relation lists can be modified using Tietze transformations (<Ref
        Sect="Changing Tietze transformation objects"/>).
      <P/>

      When a <C>IsStzPresentation</C> object <A>stz</A> is created from an fp
      semigroup <C>s</C> using <C>stz := StzPresentation(s)</C>, the generators
      and relations of <A>stz</A> are initially equal to the generators and
      relations of <C>s</C>. However, as the Stz object <A>stz</A> is modified,
      these lists may change, and their current state can be viewed using
      <Ref Oper="GeneratorsOfStzPresentation"/> and
      <Ref Oper="RelationsOfStzPresentation"/>.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");;
gap> AssignGeneratorVariables(F);;
gap> S := F / [[a * b, c], [b * c, a], [c * a, b]];
<fp semigroup with 3 generators and 3 relations of length 12>
gap> stz := StzPresentation(S);
<fp semigroup presentation with 3 generators and 3 relations
 with length 12>
gap> IsStzPresentation(stz);
true]]></Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="SimplifiedFpSemigroup">
  <ManSection>
    <Oper Name="SimplifiedFpSemigroup" Arg="S"/>
    <Returns>
      A finitely presented semigroup.
    </Returns>
    <Description>
      If <A>S</A> is an fp semigroup object (<Ref Filt="IsFpSemigroup"
      BookName="ref"/>), then <C>SimplifiedFpSemigroup</C> will return an
      FpSemigroup object <A>T</A> which is isomorphic to <A>S</A> which has
      been reduced to minimise its length.

      <C>SimplifiedFpSemigroup</C> applies <Ref Oper="SimplifyFpSemigroup"/>
      and assigns the <C>Range</C> of the isomorphism object which is returned
      to <A>T</A>, adding the isomorphism to <A>T</A> as an attribute. In this
      way, while <A>T</A> is a completely new FpSemigroup object, words in
      <A>S</A> can be mapped to <A>T</A> using the map obtained from the
      attribute <Ref Oper="FpTietzeIsomorphism"/>.<P/>

      For more information on the mapping between the semigroups and how it is
      created, see
      <Ref Sect="Converting a Tietze transformation object into a fp semigroup"/>.

      <Log><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");;
gap> S := F / [[F.1 ^ 4, F.1], [F.1, F.1 ^ 44], [F.1 ^ 8, F.2 * F.3]];;
gap> T := SimplifiedFpSemigroup(S);;
#I  Applying StzSimplifyPresentation...
#I  StzSimplifyPresentation is verbose by default. Use SetInfoLevel(InfoFpSemigroup, 1) to hide
#I  output while maintaining ability to use StzPrintRelations, StzPrintGenerators, etc.
#I  Current: <fp semigroup presentation with 3 generators and 3 relations with length 63>
#I  <Replacing all instances in other relations of relation: 1. a^4 = a>
#I  Current: <fp semigroup presentation with 3 generators and 3 relations with length 24>
#I  <Replacing all instances in other relations of relation: 1. a^4 = a>
#I  Current: <fp semigroup presentation with 3 generators and 3 relations with length 18>
#I  <Replacing all instances in other relations of relation: 1. a^4 = a>
#I  Current: <fp semigroup presentation with 3 generators and 3 relations with length 15>
#I  <Replacing all instances in other relations of relation: 3. a = a^2>
#I  Current: <fp semigroup presentation with 3 generators and 3 relations with length 12>
#I  <Removing duplicate relation: 1. a = a^2>
#I  Current: <fp semigroup presentation with 3 generators and 2 relations with length 9>
#I  <Removing redundant generator a using relation : 2. a = b*c>
#I  Current: <fp semigroup presentation with 2 generators and 1 relation with length 8>
gap> map := FpTietzeIsomorphism(T);;
gap> S.1 ^ map;
b*c
gap> S.1 ^ map = T.1 * T.2;
true
gap> invmap := InverseGeneralMapping(map);;
gap> T.1 ^ invmap = S.2;
true
gap> T.1 = S.2;
false]]></Log>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="SimplifyFpSemigroup">
  <ManSection>
    <Oper Name="SimplifyFpSemigroup" Arg="S"/>
    <Returns>
      A mapping object.
    </Returns>
    <Description>
      If <A>S</A> is a finitely presented semigroup, then
      <C>SimplifyFpSemigroup</C>
      will return a mapping object which will map <A>S</A> to a finitely
      presented semigroup
      which has had its presentation simplified.

      <C>SimplifyFpSemigroup</C> creates an <C>StzPresentation</C> object
      <A>stz</A> from <A>S</A>, which is then reduced using Tietze
      transformations until the presentation cannot be reduced in length any
      further.<P/>

      <C>SimplifyFpSemigroup</C> applies the function
      <Ref Oper="StzSimplifyPresentation"/>
      to <A>stz</A>, which repeatedly checks
      whether a number of different possible transformations will cause a
      reduction in length, and if so applies the best one. This loop continues
      until no transformations cause any reductions, in which case
      the mapping is returned. The newly reduced FpSemigroup can be accessed
      either by taking the range of the mapping or calling
      <C>SimplifiedFpSemigroup</C>, which first runs <C>SimplifyFpSemigroup</C>
      and then returns the range of the mapping with the mapping held as an
      attribute.<P/>

      For more information on how the mapping is created and used, go to
      <Ref Sect="Converting a Tietze transformation object into a fp semigroup"/>.

      <Log><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup on the generators [ a, b, c ]>
gap> map := SimplifyFpSemigroup(T);;
#I  Applying StzSimplifyPresentation...
#I  StzSimplifyPresentation is verbose by default. Use SetInfoLevel(InfoFpSemigroup, 1) to hide
#I  output while maintaining ability to use StzPrintRelations, StzPrintGenerators, etc.
#I  Current: <fp semigroup presentation with 3 generators and 2 relations with length 19>
#I  <Removing redundant generator a using relation : 1. a = b^5*c>
#I  Current: <fp semigroup presentation with 2 generators and 1 relation with length 11>
#I  <Creating new generator to replace instances of word: b^3>
#I  Current: <fp semigroup presentation with 3 generators and 2 relations with length 10>
gap> IsMapping(map);
true
gap> T.1;
a
gap> T.1 ^ map;
b^5*c
gap> RelationsOfFpSemigroup(Range(map));
[ [ b^3, d ], [ d^2, d ] ]]]></Log>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzSimplifyOnce">
  <ManSection>
    <Oper Name="StzSimplifyOnce" Arg="stz"/>
    <Returns>
      <K>true</K> or <K>false</K>.
    </Returns>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>StzSimplifyOnce</C> will check the possible reductions in length for a
      number of different possible Tietze transformations, and apply the choice
      which gives the least length. If a valid transformation was found then
      the function returns <K>true</K>, and if no transformation was performed
      because none would lead to a reduction in length, then the function
      returns <K>false</K>.<P/>

      There are four different possible transformations that
      <C>StzSimplifyOnce</C> may apply. The function searches for redundant
      generators and checks if removing them would reduce the overall length of
      the presentation, it checks whether substituting one side of each
      relation throughout the rest of the relations would reduce the length, it
      checks whether there are any trivial relations (of the form <A>w = w</A>
      for some word <A>w</A>) or any duplicated relations (relations which are
      formed from precisely the same words as another relation), and it checks
      whether any frequently occurring subwords in the relations can be
      replaced with a new generator to reduce the length. For more details, see
      <Ref Sect="Changing Tietze transformation objects"/><P/>

      At <C>InfoLevel</C> 2 (which is the default value, and which can be set
      using <C>SetInfoLevel(InfoFpSemigroup, 2)</C>), the precise transformations
      performed are printed to the screen.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup with 3 generators and 2 relations of length 19>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 2 relations
 with length 19>
gap> StzSimplifyOnce(stz);
#I  <Removing redundant generator a using relation : 1. a = b^5*c>
true
gap> stz;
<fp semigroup presentation with 2 generators and 1 relation
 with length 11>]]></Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzSimplifyPresentation">
  <ManSection>
    <Oper Name="StzSimplifyPresentation" Arg="stz"/>
    <Description>
      If <A>stz</A> is an <C>StzPresentation</C> object, then
      <C>StzSimplifyPresentation</C> will repeatedly apply the best of a few
      possible reductions to <A>stz</A> until it can no longer reduce the
      length of the presentation.<P/>
      <Log><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup on the generators [ a, b, c ]>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 2 relations
 with length 19>
gap> StzSimplifyPresentation(stz);
gap> RelationsOfStzPresentation(stz);
[ [ [ 1, 1, 1 ], [ 3 ] ], [ [ 3, 3 ], [ 3 ] ] ]]]></Log>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="UnreducedFpSemigroup-S">
  <ManSection>
    <Attr Name="UnreducedFpSemigroup" Arg="S" Label="for a semigroup"/>
    <Returns>
      <A>T</A>, an fp semigroup object.
    </Returns>
    <Description>
      If <A>S</A> is an fp semigroup object that has been obtained through
      calling <Ref Oper="SimplifiedFpSemigroup"/> on some fp semigroup <A>T</A>
      then <C>UnreducedFpSemigroup</C> returns the original semigroup object
      before simplification. These are unrelated semigroup objects, except that
      <A>S</A> will have a <Ref Oper="FpTietzeIsomorphism"/> attribute that
        returns an isomorphic mapping from <A>T</A> to <A>S</A>.
      <P/>

      If <Ref Oper="SimplifyFpSemigroup"/> has been called on an fp semigroup
      <A>T</A>, then <C>UnreducedFpSemigroup</C> can be used on the
      <C>Range</C> of the resultant mapping to obtain the domain.
      <Log><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup on the generators [ a, b, c ]>
gap> S := SimplifiedFpSemigroup(T);
#I  Applying StzSimplifyPresentation...
#I  StzSimplifyPresentation is verbose by default. Use SetInfoLevel(InfoFpSemigroup, 1) to hide
#I  output while maintaining ability to use StzPrintRelations, StzPrintGenerators, etc.
#I  Current: <fp semigroup presentation with 3 generators and 2 relations with length 19>
#I  <Removing redundant generator a using relation : 1. a = b^5*c>
#I  Current: <fp semigroup presentation with 2 generators and 1 relation with length 11>
#I  <Creating new generator to replace instances of word: b^3>
#I  Current: <fp semigroup presentation with 3 generators and 2 relations with length 10>
<fp semigroup on the generators [ b, c, d ]>
gap> UnreducedFpSemigroup(S) = T;
true]]></Log>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="FpTietzeIsomorphism">
  <ManSection>
    <Attr Name="FpTietzeIsomorphism" Arg="S"/>
    <Returns>
      A mapping object.
    </Returns>
    <Description>
      If <A>S</A> is an fp semigroup object that has been obtained through
      calling <Ref Oper="SimplifiedFpSemigroup"/> on some fp semigroup
      <A>T</A>, then <C>FpTietzeIsomorphism</C> returns an isomorphism from
      <A>T</A> to <A>S</A>. Simplification produces an fp semigroup isomorphic
      to the original fp semigroup, and these two fp semigroup objects can
      interact with each other through the mapping given by this function.
      <Log><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup on the generators [ a, b, c ]>
gap> S := SimplifiedFpSemigroup(T);
#I  Applying StzSimplifyPresentation...
#I  StzSimplifyPresentation is verbose by default. Use SetInfoLevel(InfoFpSemigroup, 1) to hide
#I  output while maintaining ability to use StzPrintRelations, StzPrintGenerators, etc.
#I  Current: <fp semigroup presentation with 3 generators and 2 relations with length 19>
#I  <Removing redundant generator a using relation : 1. a = b^5*c>
#I  Current: <fp semigroup presentation with 2 generators and 1 relation with length 11>
#I  <Creating new generator to replace instances of word: b^3>
#I  Current: <fp semigroup presentation with 3 generators and 2 relations with length 10>
<fp semigroup on the generators [ b, c, d ]>
gap> T.2;
b
gap> S.1;
b
gap> T.2 = S.1;
false
gap> map := FpTietzeIsomorphism(S);;
gap> T.2 ^ map = S.1;
true]]></Log>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="GeneratorsOfStzPresentation">
  <ManSection>
    <Attr Name="GeneratorsOfStzPresentation" Arg="stz"/>
    <Returns>
      A list of strings.
    </Returns>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>GeneratorsOfStzPresentation</C> will return (as strings) the
      generators of the fp semigroup that the presentation was created from. In
      the <Ref Oper="StzPresentation"/> object, it is only necessary to know
      how many generators there are, but for the purposes of representing
      generators and relations of the presentation object and building a new fp
      semigroup from the object, the strings representing the generators are
      stored.
      <P/>

      As Tietze transformations are performed on <A>stz</A>, the generators
      will change, but the labels will remain as close to the original labels
      as possible, so that if a generator in the fp semigroup obtained from the
      presentation is the same as a generator in the original fp semigroup,
      then they should have the same label.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup with 3 generators and 2 relations of length 19>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 2 relations
 with length 19>
gap> GeneratorsOfStzPresentation(stz);
[ "a", "b", "c" ]]]></Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="RelationsOfStzPresentation">
  <ManSection>
    <Attr Name="RelationsOfStzPresentation" Arg="stz"/>
    <Returns>
      A list of pairs of words in <C>LetterRep</C>
      (<Ref Oper="LetterRepAssocWord" BookName="ref"/>)
      form.
    </Returns>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>RelationsOfStzPresentation</C> will return in letter rep form the
      current relations of the presentation object. When the presentation
      object is first created, these will be the <C>LetterRep</C> forms of the
      relations of the fp semigroup object that is used to create <A>stz</A>.
      As Tietze transformations are performed on the presentation object, the
      relations returned by this function will change to reflect the
      transformations.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup with 3 generators and 2 relations of length 19>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 2 relations
 with length 19>
gap> RelationsOfStzPresentation(stz);
[ [ [ 1 ], [ 2, 2, 2, 2, 2, 3 ] ],
  [ [ 2, 2, 2, 2, 2, 2 ], [ 2, 2, 2 ] ] ]]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="UnreducedFpSemigroup-stz">
  <ManSection>
    <Attr Name="UnreducedFpSemigroup" Arg="stz" Label="for a presentation"/>
    <Returns>
      An fp semigroup.
    </Returns>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>UnreducedFpSemigroup</C> will return the fp semigroup
      that was used to create <A>stz</A> using <Ref Oper="StzPresentation"/>.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup with 3 generators and 2 relations of length 19>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 2 relations
 with length 19>
gap> UnreducedFpSemigroup(stz) = T;
true]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzAddRelation">
  <ManSection>
    <Oper Name="StzAddRelation" Arg="stz, pair"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object and <A>pair</A>
      is a list containing two <C>LetterRep</C> words over the generators of
      <A>stz</A>, then <C>StzAddRelation</C> will perform a Tietze
      transformation of the first type and add a new relation to <A>stz</A>.
      This only happens if the new relation that would be formed from
      <A>pair</A> can be constructed from the other existing relations; that
      is, if we can perform elementary operations using the existing relations
        of <A>stz</A> to convert <C><A>pair</A>[1]</C> into
        <C><A>pair</A>[2]</C>.
      <P/>

      If, instead, <A>pair</A> is a list containing two elements of the fp
      semigroup <C>S</C> that was used to create <C>stz</C>, and the two words
      are equal in that semigroup, then this function
      will add the <C>LetterRep</C> of these words as a new relation to
      <A>stz</A>.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3]];
<fp semigroup with 3 generators and 2 relations of length 19>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 2 relations
 with length 19>
gap> pair := [[2, 2, 2, 2, 2, 2, 2, 2, 2], [2, 2, 2]];;
gap> StzAddRelation(stz, pair);
gap> RelationsOfStzPresentation(stz);
[ [ [ 1 ], [ 2, 2, 2, 2, 2, 3 ] ],
  [ [ 2, 2, 2, 2, 2, 2 ], [ 2, 2, 2 ] ],
  [ [ 2, 2, 2, 2, 2, 2, 2, 2, 2 ], [ 2, 2, 2 ] ] ]
gap> pair2 := [[1, 1], [3]];
[ [ 1, 1 ], [ 3 ] ]
gap> StzAddRelation(stz, pair2);
Error, StzAddRelation: second argument <pair> must list two
words that are equal in the presentation <stz>]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzRemoveRelation">
  <ManSection>
    <Oper Name="StzRemoveRelation" Arg="stz, index"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object and <A>index</A>
      is a positive integer less than or equal to the number of relations of
      <A>stz</A>, then <C>StzRemoveRelation</C> will perform a Tietze
      transformation of the second type and remove the <A>index</A>th relation
      of <A>stz</A> if that relation is such that one side of it can be
      obtained from the other by a sequence of elementary operations using only
        the other relations of <A>stz</A>.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> StzRemoveRelation(stz, 2);
gap> RelationsOfStzPresentation(stz);
[ [ [ 1 ], [ 2, 2, 2, 2, 2, 3 ] ], [ [ 2, 2 ], [ 2 ] ] ]
gap> StzRemoveRelation(stz, 2);
Error, StzRemoveRelation: second argument <index> must point to
a relation that is redundant in the presentation <stz>]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzAddGenerator">
  <ManSection>
    <Oper Name="StzAddGenerator" Arg="stz, word[, name]"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object and <A>word</A>
      is a <C>LetterRep</C> word over the generators of <A>stz</A>, then
      <C>StzAddGenerator</C> will perform a Tietze transformation  which adds a
      new generator to <A>stz</A> and a new relation of the form
      <C>newgenerator = <A>word</A></C>.
      <P/>

      If, instead, <A>word</A> is a word over the fp semigroup <A>S</A> that
      was used to create <A>stz</A>, then this function will add the new
      generator and a new relation with the new generator equal to the
      <C>LetterRep</C> of this word as a new relation to <A>stz</A>.
      <P/>

      A new name for the generator is chosen and added automatically based on
      the names of the existing generators to <Ref
        Attr="GeneratorsOfStzPresentation"/> if the argument <A>name</A> is not
      included. If it is, and if <A>name</A> is a string that is not equal to
      an existing generator, then the string added to the list of generators
      will be <A>name</A> instead.
      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> StzAddGenerator(stz, [2, 2, 2]);
gap> RelationsOfStzPresentation(stz);
[ [ [ 1 ], [ 2, 2, 2, 2, 2, 3 ] ],
  [ [ 2, 2, 2, 2, 2, 2 ], [ 2, 2, 2 ] ], [ [ 2, 2 ], [ 2 ] ],
  [ [ 2, 2, 2 ], [ 4 ] ] ]]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzRemoveGenerator">
  <ManSection>
    <Oper Name="StzRemoveGenerator" Arg="stz, gen/genname[, index]"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object
      and <A>gen</A> is a positive
      integer less than or equal to the number of generators of <A>stz</A>, then
      <C>StzRemoveGenerator</C> will perform a Tietze transformation which
      removes the <A>gen</A>th generator of <A>stz</A>.<P/>

      The argument <A>stz</A>
      must contain a relation of the form <C><A>gen</A> = word</C> or
      <C>word = <A>gen</A></C>, where <C>word</C> contains no occurrences of
      the generator <A>gen</A> being removed. The generator is then removed
      from the presentation by replacing every instance of <A>gen</A> with a
      copy of <C>word</C>.<P/>

      If the second argument is a string <A>genname</A> rather than a positive
      integer <A>gen</A>, then the function searches the generators of
      <A>stz</A> for a generator with the same name and attempts to remove the
      generator if the same conditions as above are met.
      <P/>

      If the argument <A>index</A> is included and is a positive integer less
      than or equal to the number of relations, then rather than searching the
      relations for the first to satisfy the necessary conditions, the function
      checks the <A>index</A>th relation to see if it satisfies those
      conditions, and applies the Tietze transformation by removing this
      relation.
      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> StzRemoveGenerator(stz, 1);
gap> RelationsOfStzPresentation(stz);
[ [ [ 1, 1, 1, 1, 1, 1 ], [ 1, 1, 1 ] ], [ [ 1, 1 ], [ 1 ] ] ]]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzSubstituteRelation">
  <ManSection>
    <Oper Name="StzSubstituteRelation" Arg="stz, index, side"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object and <A>index</A>
      is a positive integer less than or equal to the number of relations of
      <A>stz</A> and <A>side</A> is either <C>1</C> or <C>2</C>, then
      <C>StzRemoveGenerator</C> will perform a sequence of Tietze
      transformations in order to replace, for the <A>index</A>th relation (say
      <C>[u, v]</C>), to replace all instances of the <A>side</A>th word of the
      relation in all other relations by the other side (so, for
      <C><A>side</A>=1</C>, all instances of <C>u</C> in all other relations of
      <A>stz</A> are replaced by <C>v</C>). This requires two Tietze
      transformations per relation containing <C>u</C>, one to add a new
      redundant relation with each <C>u</C> replaced by <C>v</C>, and another
      to remove the original (now redundant) relation.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> StzSubstituteRelation(stz, 3, 1);
gap> RelationsOfStzPresentation(stz);
[ [ [ 1 ], [ 2, 2, 2, 3 ] ], [ [ 2, 2, 2 ], [ 2, 2 ] ],
  [ [ 2, 2 ], [ 2 ] ] ]
gap> StzSubstituteRelation(stz, 3, 1);
gap> RelationsOfStzPresentation(stz);
[ [ [ 1 ], [ 2, 2, 3 ] ], [ [ 2, 2 ], [ 2 ] ], [ [ 2, 2 ], [ 2 ] ] ]]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="Length">
  <ManSection>
    <Oper Name="Length" Arg="stz"/>
    <Returns>
      A non-negative integer.
    </Returns>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then the
      <C>Length</C> of the object is defined as the number of generators plus
      the lengths of each word in each relation of <A>stz</A>.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> Length(stz);
22]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzPrintRelation">
  <ManSection>
    <Oper Name="StzPrintRelation" Arg="stz, int"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>StzPrintRelation</C> calls <C>StzPrintRelations</C> with parameters
      <A>stz</A> and <C>[<A>int</A>]</C>.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> StzPrintRelation(stz, 2);
#I  2. b^6 = b^3]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzPrintRelations">
  <ManSection>
    <Oper Name="StzPrintRelations" Arg="stz[, list]"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object and <A>list</A>
      is a list of positive integers, then <C>StzPrintRelations</C> prints for
      each <A>i</A> in <A>list</A> the <A>i</A>th relation to the console in
      terms of the stored labels for the generators (that is, as words over the
      alphabet consisting of the generators of <A>stz</A>).<P/>

      If <A>list</A> is not specified then <C>StzPrintRelations</C> prints all
      relations of <A>stz</A> in order.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> StzPrintRelations(stz, [2, 3]);
#I  2. b^6 = b^3
#I  3. b^2 = b
gap> StzPrintRelations(stz);
#I  1. a = b^5*c
#I  2. b^6 = b^3
#I  3. b^2 = b]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzPrintGenerators">
  <ManSection>
    <Oper Name="StzPrintGenerators" Arg="stz[, list]"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object and <A>list</A>
      is a list of positive integers, then <C>StzPrintGenerators</C> for each
      <A>i</A> in <A>list</A> the <A>i</A>th generator and the number of
      occurrences of that generator in the relations is printed to the
      screen.<P/>

      If <A>list</A> is not specified then <C>StzPrintGenerators</C> prints all
      generators of <A>stz</A> in order.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> StzPrintGenerators(stz, [1, 2]);
#I  1.  a  1 occurrences
#I  2.  b  17 occurrences
gap> StzPrintGenerators(stz);
#I  1.  a  1 occurrences
#I  2.  b  17 occurrences
#I  3.  c  1 occurrences]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzPrintPresentation">
  <ManSection>
    <Oper Name="StzPrintPresentation" Arg="stz"/>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>StzPrintPresentation</C> prints a comprehensive overview of
      <A>stz</A>, including the generators and number of occurrences of each
      generator in the relations, the relations as words over the generators,
      and the forward and backward maps that indicate how the unreduced
        semigroup maps to the semigroup currently described by <A>stz</A>.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> T := F / [[F.1, F.2 ^ 5 * F.3],
>              [F.2 ^ 6, F.2 ^ 3], [F.2 ^ 2, F.2]];
<fp semigroup with 3 generators and 3 relations of length 22>
gap> stz := StzPresentation(T);
<fp semigroup presentation with 3 generators and 3 relations
 with length 22>
gap> StzPrintPresentation(stz);
#I  Current generators:
#I  1.  a  1 occurrences
#I  2.  b  17 occurrences
#I  3.  c  1 occurrences
#I
#I  Current relations:
#I  1. a = b^5*c
#I  2. b^6 = b^3
#I  3. b^2 = b
#I
#I  There are 3 generators and 3 relations of total length 22.
#I
#I  Generators of original fp semigroup expressed as
#I  combinations of generators in current presentation:
#I  1. a = a
#I  2. b = b
#I  3. c = c
#I
#I  Generators of current presentation expressed as
#I  combinations of generators of original fp semigroup:
#I  1. a = a
#I  2. b = b
#I  3. c = c]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="TietzeForwardMap">
  <ManSection>
    <Attr Name="TietzeForwardMap" Arg="stz"/>
    <Returns>
      A list of lists of positive integers.
    </Returns>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>TietzeForwardMap</C> returns a list of lists of positive integers.
      There is an element of this list for every generator of the unreduced
      semigroup of the presentation (see <Ref Attr="UnreducedFpSemigroup"
        Label="for a presentation"/>) that indicates the word (in
      <C>LetterRep</C> form) in the semigroup object currently defined by the
      presentation that the generator maps to.<P/>

      This mapping is updated as the presentation object is transformed. It
      begins as a list of the form <C>[[1], [2], [3], . . ., [n]]</C> where
      <C>n</C> is the number of generators of the unreduced semigroup.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> S := F / [[F.1, F.3 ^ 3], [F.2 ^ 2, F.3 ^ 2]];
<fp semigroup with 3 generators and 2 relations of length 11>
gap> stz := StzPresentation(S);
<fp semigroup presentation with 3 generators and 2 relations
 with length 11>
gap> StzRemoveGenerator(stz, 1);
gap> TietzeForwardMap(stz);
[ [ 2, 2, 2 ], [ 1 ], [ 2 ] ]]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="TietzeBackwardMap">
  <ManSection>
    <Attr Name="TietzeBackwardMap" Arg="stz"/>
    <Returns>
      A list of lists of positive integers.
    </Returns>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>TietzeBackwardMap</C> returns a list of lists of positive integers.
      There is an element of this list for every generator of the semigroup
      that the presentation currently defines that indicates the word (in
      <C>LetterRep</C> form) in the unreduced semigroup of the presentation
      (see <Ref Attr="UnreducedFpSemigroup" Label="for a presentation"/>) that
      the generator maps to.<P/>

      This mapping is updated as the presentation object is transformed. It
      begins as a list of the form <C>[[1], [2], [3], . . ., [n]]</C> where
      <C>n</C> is the number of generators of the unreduced semigroup.

      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> S := F / [[F.1, F.3 ^ 3], [F.2 ^ 2, F.3 ^ 2]];
<fp semigroup with 3 generators and 2 relations of length 11>
gap> stz := StzPresentation(S);
<fp semigroup presentation with 3 generators and 2 relations
 with length 11>
gap> StzRemoveGenerator(stz, 1);
gap> TietzeBackwardMap(stz);
[ [ 2 ], [ 3 ] ]]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>

<#GAPDoc Label="StzIsomorphism">
  <ManSection>
    <Oper Name="StzIsomorphism" Arg="stz"/>
    <Returns>
      A mapping object.
    </Returns>
    <Description>
      If <A>stz</A> is an <Ref Oper="StzPresentation"/> object, then
      <C>StzIsomorphism</C> returns a mapping object that maps the unreduced
      semigroup of the presentation (see <Ref Attr="UnreducedFpSemigroup"
        Label="for a presentation"/>) to an FpSemigroup object that is defined
        by the generators and relations of the semigroup presentation at the
      moment this function is ran.<P/>

      If a <C>StzIsomorphism</C> is generated from <A>stz</A>, and the
      presentation <A>stz</A> is further modified afterwards (for example by
      applying more Tietze transformations or <Ref Oper="StzSimplifyOnce"/> to
      <A>stz</A>), then running <C>StzIsomorphism(<A>stz</A>)</C> a second
      time will produce a different result consistent with the <E>new</E>
      generators and relations of <A>stz</A>.<P/>

      This mapping is built from the <Ref Attr="TietzeForwardMap"/> and <Ref
      Attr="TietzeBackwardMap"/> attributes from the presentation object, since
      if we know how to map the generators of the respective semigroups, then
      we know how to map any element of that semigroup.<P/>

      This function is the primary way to obtain the simplified semigroup from
      the presentation object, by applying <C>Range</C> to the mapping that
      this function returns.
      <Example><![CDATA[
gap> F := FreeSemigroup("a", "b", "c");
<free semigroup on the generators [ a, b, c ]>
gap> S := F / [[F.1, F.3 ^ 3], [F.2 ^ 2, F.3 ^ 2]];
<fp semigroup with 3 generators and 2 relations of length 11>
gap> stz := StzPresentation(S);
<fp semigroup presentation with 3 generators and 2 relations
 with length 11>
gap> StzRemoveGenerator(stz, "a");
gap> map := StzIsomorphism(stz);
<fp semigroup with 3 generators and 2 relations of length 11> ->
<fp semigroup with 2 generators and 1 relation of length 6>
gap> S.1 ^ map;
c^3]]>
</Example>
    </Description>
  </ManSection>
<#/GAPDoc>
