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#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Isabel Araújo.
##
## Copyright of GAP belongs to its developers, whose names are too numerous
## to list here. Please refer to the COPYRIGHT file for details.
##
## SPDX-License-Identifier: GPL-2.0-or-later
##
## This file contains the declarations for semigroups defined by rws.
##

############################################################################
##
#A ReducedConfluentRewritingSystem( <S>[, <ordering>] )
##
## <#GAPDoc Label="ReducedConfluentRewritingSystem">
## <ManSection>
## <Attr Name="ReducedConfluentRewritingSystem" Arg='S[, ordering]'/>
##
## <Description>
## returns a reduced confluent rewriting system of
## the finitely presented semigroup or monoid <A>S</A> with respect to the
## reduction ordering <A>ordering</A> (see <Ref Chap="Orderings"/>).
## 
## The default for <A>ordering</A> is the length plus lexicographic ordering
## on words, also called the shortlex ordering; for the definition see for
## example <Cite Key="Sims94"/>.
## 
## Notice that this might not terminate. In particular, if the semigroup or
## monoid <A>S</A> does not have a solvable word problem then it this will
## certainly never end.
## Also, in this case, the object returned is an immutable
## rewriting system, because once we have a confluent
## rewriting system for a finitely presented semigroup or monoid we do
## not want to allow it to change (as it was most probably very time
## consuming to get it in the first place). Furthermore, this is also
## an attribute storing object (see <Ref Sect="Representation"/>).
## 
## <Example><![CDATA[
## gap> f := FreeSemigroup( "a", "b" );;
## gap> a := f.1;; b := f.2;;
## gap> s := f / [ [ a*b*a, b ], [ b*a*b, a ] ];;
## gap> rws := ReducedConfluentRewritingSystem( s );
## Rewriting System for Semigroup( [ a, b ] ) with rules
## [ [ a*b*a, b ], [ b*a*b, a ], [ b*a^2, a^2*b ], [ b^2, a^2 ],
## [ a^5, a ], [ a^3*b, b*a ] ]
## gap> c := s.1;; d := s.2;;
## gap> e := (c*d^2)^3;
## (a*b^2)^3
## gap> ## ReducedForm( rws, e ); gives an error!
## gap> w := UnderlyingElement( e );
## (a*b^2)^3
## gap> ReducedForm( rws, w );
## a
## ]]></Example>
## 
## The creation of a reduced confluent rewriting system for a semigroup
## or for a monoid, in &GAP;, uses the Knuth-Bendix procedure for strings,
## which manipulates a rewriting system of the semigroup or monoid and
## attempts to make it confluent,
## (see Chapter <Ref Chap="Rewriting Systems"/>
## and also Sims <Cite Key="Sims94"/>).
## (Since the word problem for semigroups/monoids is not solvable in general,
## the Knuth-Bendix procedure cannot always terminate).
## 
## In order to apply this procedure we will build a rewriting system
## for the semigroup or monoid, which we will call a <E>Knuth-Bendix Rewriting
## System</E> (we need to define this because we need the rewriting system
## to store some information needed for the implementation of the
## Knuth-Bendix procedure).
## 
## Actually, Knuth-Bendix Rewriting Systems do not only serve this purpose.
## Indeed these are objects which are mutable and which can be manipulated
## (see <Ref Chap="Rewriting Systems"/>).
## 
## Note that the implemented version of the Knuth-Bendix procedure, in &GAP;
## returns, if it terminates, a confluent rewriting system which is reduced.
## Also, a reduction ordering has to be specified when building a rewriting
## system. If none is specified, the shortlex ordering is assumed
## (note that the procedure may terminate with a certain ordering and
## not with another one).
## 
## On Unix systems it is possible to replace the built-in Knuth-Bendix by
## other routines, for example the package <Package>kbmag</Package> offers
## such a possibility.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareAttribute("ReducedConfluentRewritingSystem",IsSemigroup);

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##
#A FreeSemigroupOfRewritingSystem(<rws>)
#A FreeMonoidOfRewritingSystem(<rws>)
##
## <#GAPDoc Label="FreeSemigroupOfRewritingSystem">
## <ManSection>
## <Attr Name="FreeSemigroupOfRewritingSystem" Arg='rws'/>
## <Attr Name="FreeMonoidOfRewritingSystem" Arg='rws'/>
##
## <Description>
## returns the free semigroup or monoid over which <A>rws</A> is
## a rewriting system.
## 
## <Example><![CDATA[
## gap> f1 := FreeSemigroupOfRewritingSystem( rws );
## <free semigroup on the generators [ a, b ]>
## gap> f1 = f;
## true
## gap> s1 := SemigroupOfRewritingSystem( rws );
## <fp semigroup on the generators [ a, b ]>
## gap> s1 = s;
## true
## ]]></Example>
## 
## As mentioned before, having a confluent rewriting system, one can decide
## whether two words represent the same element of a finitely
## presented semigroup (or finitely presented monoid).
## 
## <Example><![CDATA[
## gap> d^6 = c^2;
## true
## gap> ReducedForm( rws, UnderlyingElement( d^6 ) );
## a^2
## gap> ReducedForm( rws, UnderlyingElement( c^2 ) );
## a^2
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareAttribute("FreeSemigroupOfRewritingSystem", IsRewritingSystem);
DeclareAttribute("FreeMonoidOfRewritingSystem", IsRewritingSystem);

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##
#A FamilyForRewritingSystem(<rws>)
##
## <ManSection>
## <Attr Name="FamilyForRewritingSystem" Arg='rws'/>
##
## <Description>
## returns the family of words over which <A>rws</A> is
## a rewriting system
## </Description>
## </ManSection>
##
DeclareAttribute("FamilyForRewritingSystem", IsRewritingSystem);

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##
#F ReduceLetterRepWordsRewSys(<tzrules>,<w>)
##
## <ManSection>
## <Func Name="ReduceLetterRepWordsRewSys" Arg='tzrules,w'/>
##
## <Description>
## Here <A>w</A> is a word of a free monoid or a free semigroup in tz
## representation, and <A>tzrules</A> are rules in tz representation.
## This function returns the reduced word in tz representation.
## 
## All lists in <A>tzrules</A> as well as <A>w</A> must be plain lists,
## the entries must be small integers.
## (The behaviour otherwise is unpredictable.)
## </Description>
## </ManSection>
##
DeclareGlobalFunction("ReduceLetterRepWordsRewSys");

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