  
  [1X2 [33X[0;0YRewriting Systems[133X[101X
  
  [33X[0;0YThis chapter describes functions to construct rewriting systems for finitely
  presented  groups  which  store rewriting information. The main example used
  throughout  this  manual  is  a  presentation  of  the quaternion group [22Xq8 =
  F/[a^4, b^4, abab^-1, a^2b^2][122X.[133X
  
  
  [1X2.1 [33X[0;0YMonoid Presentations of FpGroups[133X[101X
  
  [1X2.1-1 FreeRelatorGroup[101X
  
  [33X[1;0Y[29X[2XFreeRelatorGroup[102X( [3Xgrp[103X ) [32X attribute[133X
  [33X[1;0Y[29X[2XFreeRelatorHomomorphism[102X( [3Xgrp[103X ) [32X attribute[133X
  
  [33X[0;0YThe function [10XFreeRelatorGroup[110X returns a free group on the set of relators of
  the  fp-group [10XG[110X. If [10XHasName(G)[110X is [9Xtrue[109X then a name is automatically assigned
  to this free group by concatenating [10X_R[110X.[133X
  
  [33X[0;0YThe function [10XFreeRelatorHomomorphism[110X returns the group homomorphism from the
  free group on the relators to the free group on the generators of [10XG[110X, mapping
  each generator to the corresponding word.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xrelq8 := [ f^4, g^4, f*g*f*g^-1, f^2*g^2 ];;[127X[104X
    [4X[25Xgap>[125X [27Xq8 := F/relq8;;[127X[104X
    [4X[25Xgap>[125X [27XSetName( q8, "q8" );;[127X[104X
    [4X[25Xgap>[125X [27Xq8R := FreeRelatorGroup( q8 ); [127X[104X
    [4X[28Xq8_R[128X[104X
    [4X[25Xgap>[125X [27Xgenq8R := GeneratorsOfGroup( q8R );[127X[104X
    [4X[28X[ q8_R1, q8_R2, q8_R3, q8_R4][128X[104X
    [4X[25Xgap>[125X [27Xhomq8R := FreeRelatorHomomorphism( q8 );[127X[104X
    [4X[28X[ q8_R1, q8_R2, q8_R3, q8_R4 ] -> [ f1^4, f2^4, f1*f2*f1*f2^-1, f1^2*f2^2 ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [1X2.1-2 MonoidPresentationFpGroup[101X
  
  [33X[1;0Y[29X[2XMonoidPresentationFpGroup[102X( [3Xgrp[103X ) [32X attribute[133X
  [33X[1;0Y[29X[2XArrangementOfMonoidGenerators[102X( [3Xgrp[103X ) [32X attribute[133X
  [33X[1;0Y[29X[2XMonoidPresentationLabels[102X( [3Xgrp[103X ) [32X attribute[133X
  [33X[1;0Y[29X[2XFreeGroupOfPresentation[102X( [3Xmon[103X ) [32X attribute[133X
  [33X[1;0Y[29X[2XGroupRelatorsOfPresentation[102X( [3Xmon[103X ) [32X attribute[133X
  [33X[1;0Y[29X[2XInverseRelatorsOfPresentation[102X( [3Xmon[103X ) [32X attribute[133X
  [33X[1;0Y[29X[2XHomomorphismOfPresentation[102X( [3Xmon[103X ) [32X attribute[133X
  
  [33X[0;0YA  monoid  presentation  for  a  finitely  presented  group [10XG[110X has two monoid
  generators  [22Xg,G[122X  for  each  group  generator  [22Xg[122X.  The relators of the monoid
  presentation comprise the group relators, and relators [22XgG, Gg[122X specifying the
  inverses.   The   function   [10XMonoidPresentationFpGroup[110X  returns  the  monoid
  presentation derived in this way from an fp-presentation.[133X
  
  [33X[0;0YThe  function  [10XFreeGroupOfPresentation[110X  returns the free group on the monoid
  generators.[133X
  
  [33X[0;0YThe  function  [10XGroupRelatorsOfPresentation[110X  returns  those  relators  of the
  monoid which correspond to the relators of the group. All negative powers in
  the group relators are converted to positive powers of the [22XG[122X's. The function
  [10XInverseRelatorsOfPresentation[110X  returns  relators  which  specify the inverse
  pairs of the monoid generators.[133X
  
  [33X[0;0YThe  function  [10XHomomorphismOfPresentation[110X  returns the homomorphism from the
  free  group  of  the  monoid  presentation  to  the  free group of the group
  presentation.[133X
  
  [33X[0;0YThe  attribute  [10XArrangementOfMonoidGenerators[110X  will  be discussed before the
  second example in the next section.[133X
  
  [33X[0;0YIn  the  example  below, the four monoid generators [22Xa,b,A,B[122X are named [10Xq8_M1,
  q8_M2, q8_M3, q8_M4[110X respectively.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xmq8 := MonoidPresentationFpGroup( q8 );[127X[104X
    [4X[28Xmonoid presentation with group relators [128X[104X
    [4X[28X[ q8_M1^4, q8_M2^4, q8_M1*q8_M2*q8_M1*q8_M4, q8_M1^2*q8_M2^2 ][128X[104X
    [4X[25Xgap>[125X [27Xfmq8 := FreeGroupOfPresentation( mq8 ); [127X[104X
    [4X[28X<free group on the generators [ q8_M1, q8_M2, q8_M3, q8_M4 ]>[128X[104X
    [4X[25Xgap>[125X [27Xgenfmq8 := GeneratorsOfGroup( fmq8 );;[127X[104X
    [4X[25Xgap>[125X [27Xgprels := GroupRelatorsOfPresentation( mq8 ); [127X[104X
    [4X[28X[ q8_M1^4, q8_M2^4, q8_M1*q8_M2*q8_M1*q8_M4, q8_M1^2*q8_M2^2 ][128X[104X
    [4X[25Xgap>[125X [27Xinvrels := InverseRelatorsOfPresentation( mq8 ); [127X[104X
    [4X[28X[ q8_M1*q8_M3, q8_M2*q8_M4, q8_M3*q8_M1, q8_M4*q8_M2 ][128X[104X
    [4X[25Xgap>[125X [27Xhompres := HomomorphismOfPresentation( mq8 ); [127X[104X
    [4X[28X[ q8_M1, q8_M2, q8_M3, q8_M4 ] -> [ f1, f2, f1^-1, f2^-1 ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [1X2.1-3 PrintLnUsingLabels[101X
  
  [33X[1;0Y[29X[2XPrintLnUsingLabels[102X( [3Xargs[103X ) [32X function[133X
  [33X[1;0Y[29X[2XPrintUsingLabels[102X( [3Xargs[103X ) [32X function[133X
  
  [33X[0;0YThe  labels  [10Xq8_M1,  q8_M2,  q8_M3,  q8_M4[110X  are  rather unhelpful in lengthy
  output,  so  it  is  convenient to use [22X[a,b,A,B][122X as above. Then the function
  [10XPrintUsingLabels[110X  takes as input a word in the monoid, the generators of the
  monoid,  and a set of labels for these generators. This function also treats
  lists  of  words  and  lists  of  lists  in  a  similar  way.  The  function
  [10XPrintLnUsingLabels[110X does exactly the same, and then appends a newline.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xq8labs := [ "a", "b", "A", "B" ];; [127X[104X
    [4X[25Xgap>[125X [27XSetMonoidPresentationLabels( q8, q8labs );; [127X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( gprels, genfmq8, q8labs ); [127X[104X
    [4X[28X[ a^4, b^4, a*b*a*B, a^2*b^2 ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [1X2.1-4 InitialRulesOfPresentation[101X
  
  [33X[1;0Y[29X[2XInitialRulesOfPresentation[102X( [3Xmon[103X ) [32X function[133X
  
  [33X[0;0YThe initial rules for [22Xmq8[122X are the four rules of the form [22Xa^-1 -> A[122X; the four
  rules  of  the  form [22XaA -> id[122X; and the four relator rules of the form [22Xa^4 ->
  id[122X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xq0 := InitialRulesOfPresentation( mq8 );;  [127X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( q0, genfmq8, q8labs );[127X[104X
    [4X[28X[ [ a^-1, A ], [ b^-1, B ], [ A^-1, a ], [ B^-1, b ], [ a*A, id ], [128X[104X
    [4X[28X[ b*B, id ], [ A*a, id ], [ B*b, id ], [ a^4, id ], [ a^2*b^2, id ], [128X[104X
    [4X[28X[ a*b*a*B, id ], [ b^4, id ] ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  
  [1X2.2 [33X[0;0YRewriting systems for FpGroups[133X[101X
  
  [33X[0;0YThese  functions  duplicate  the  standard  Knuth Bendix functions which are
  available  in  the  [5XGAP[105X  library.  There are two reasons for this: (1) these
  functions  were  first written before the standard functions were available;
  (2)  we  require  logged  versions  of  the  functions,  and  these are most
  conveniently extended versions of the non-logged code.[133X
  
  [1X2.2-1 RewritingSystemFpGroup[101X
  
  [33X[1;0Y[29X[2XRewritingSystemFpGroup[102X( [3Xgrp[103X ) [32X attribute[133X
  
  [33X[0;0YThis  function attempts to return a complete rewrite system for the fp-group
  [10XG[110X   obtained   using   the   group's   monoid   presentation   [10Xmon[110X,  with  a
  length-lexicographical   ordering   on  the  words  in  [10Xfgmon[110X,  by  applying
  Knuth-Bendix  completion.  Such  a  rewrite  system  can be obtained for all
  finite  groups. The rewrite rules are (partially) ordered, starting with the
  inverse  relators,  followed  by  the rules which reduce the word length the
  most.[133X
  
  [33X[0;0YIn our [10Xq8[110X example there are 20 rewrite rules in the rewriting system [10Xrws[110X:[133X
  
      ┌──────────────────────────────────────────────────────────────────────────────────────────┐
      │                [22Xa^-1 -> A, quad b^-1 -> B, quad A^-1 -> a, quad B^-1 -> b,[122X                │ 
      │                  [22XaA -> id, quad bB -> id, quad Aa -> id, quad Bb -> id,[122X                  │ 
      │  [22Xba -> aB, quad b^2 -> a^2,quad bA -> ab, quad Ab -> aB, quad A^2 -> a^2,quad AB -> ab,[122X  │ 
      │ [22XBa -> ab, quad BA -> aB, quad B^2 -> a^2, quad a^3 -> a, quad a^2b -> B, quad a^2B -> b.[122X │ 
      └──────────────────────────────────────────────────────────────────────────────────────────┘
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xrws := RewritingSystemFpGroup( q8 );;[127X[104X
    [4X[25Xgap>[125X [27XLength( rws );[127X[104X
    [4X[28X20[128X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( rws, genfmq8, q8labs );[127X[104X
    [4X[28X[ [ a^-1, A ], [ b^-1, B ], [ A^-1, a ], [ B^-1, b ], [ a*A, id ], [128X[104X
    [4X[28X[ b*B, id ], [ A*a, id ], [ B*b, id ], [ b*a, a*B ], [ b^2, a^2 ], [128X[104X
    [4X[28X[ b*A, a*b ], [ A*b, a*B ], [ A^2, a^2 ], [ A*B, a*b ], [ B*a, a*b ], [128X[104X
    [4X[28X[ B*A, a*B ], [ B^2, a^2 ], [ a^3, A ], [ a^2*b, B ], [ a^2*B, b ] ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe     default    ordering    of    the    [22X2n[122X    monoid    generators    is
  [22X[g_1^+,g_2^+,...,g_n^+,g_1^-,g_2^-,...,g_n^-][122X.    In   the   case   of   the
  two-generator  abelian  group [22XT = ⟨ a,b ~|~ [a,b] ⟩[122X the Knuth-Bendix process
  starts  to  generate infinite sets of relations such as [22X{ab^ma^-1 -> b^m,~ m
  geqslant 1}[122X.[133X
  
  [33X[0;0YIf,   using  the  [10XArrangementOfMonoidGenerators[110X  function,  we  specify  the
  alternative  ordering  [22X[g_1^+,g_1^-,g_2^+,g_2^-][122X, then a finite set of rules
  is obtained.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27XT := F/[Comm(f,g)];              [127X[104X
    [4X[28X<fp group of size infinity on the generators [ f1, f2 ]>[128X[104X
    [4X[25Xgap>[125X [27XSetName( T, "T" );                   [127X[104X
    [4X[25Xgap>[125X [27XSetArrangementOfMonoidGenerators( T, [1,-1,2,-2] );[127X[104X
    [4X[25Xgap>[125X [27XTlabs := [ "x", "X", "y", "Y" ];; [127X[104X
    [4X[25Xgap>[125X [27XmT := MonoidPresentationFpGroup( T );              [127X[104X
    [4X[28Xmonoid presentation with group relators [ T_M2*T_M4*T_M1*T_M3 ][128X[104X
    [4X[25Xgap>[125X [27XfgmT := FreeGroupOfPresentation( mT );; [127X[104X
    [4X[25Xgap>[125X [27XgenfgmT := GeneratorsOfGroup( fgmT );;[127X[104X
    [4X[25Xgap>[125X [27XSetMonoidPresentationLabels( T, Tlabs );; [127X[104X
    [4X[25Xgap>[125X [27XrwsT := RewritingSystemFpGroup( T );;[127X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( rwsT, genfgmT, Tlabs );       [127X[104X
    [4X[28X[ [ x^-1, X ], [ X^-1, x ], [ y^-1, Y ], [ Y^-1, y ], [ x*X, id ], [128X[104X
    [4X[28X[ X*x, id ], [ y*Y, id ], [ Y*y, id ], [ y*x, x*y ], [ y*X, X*y ], [128X[104X
    [4X[28X[ Y*x, x*Y ], [ Y*X, X*Y ] ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe last four rules show that generators [22Xx[122X and [22Xy[122X commute.[133X
  
  [1X2.2-2 OnePassReduceWord[101X
  
  [33X[1;0Y[29X[2XOnePassReduceWord[102X( [3Xword[103X, [3Xrules[103X ) [32X operation[133X
  [33X[1;0Y[29X[2XReduceWordKB[102X( [3Xword[103X, [3Xrules[103X ) [32X operation[133X
  
  [33X[0;0YThese functions are called by the function [10XRewritingSystemFpGroup[110X.[133X
  
  [33X[0;0YAssuming  that  [10Xword[110X  is  an element of a free monoid and [10Xrules[110X is a list of
  ordered  pairs  of  such  words, the function [10XOnePassReduceWord[110X searches the
  list  of rules until it finds that the left-hand side of a [10Xrule[110X is a [10Xsubword[110X
  of  [10Xword[110X, whereupon it replaces that [10Xsubword[110X with the right-hand side of the
  matching  rule.  The  search  is  continued from the next [10Xrule[110X in [10Xrules[110X, but
  using the new [10Xword[110X. When the end of [10Xrules[110X is reached, one pass is considered
  to  have been made and the reduced [10Xword[110X is returned. If no matches are found
  then the original [10Xword[110X is returned.[133X
  
  [33X[0;0YThe  function [10XReduceWordKB[110X repeatedly applies the function [10XOnePassReduceWord[110X
  until  the [10Xword[110X remaining contains no left-hand side of a [10Xrule[110X as a [10Xsubword[110X.
  If  [10Xrules[110X  is  a  complete rewrite system, then the irreducible [10Xword[110X that is
  returned is unique, otherwise the order of the rules in [10Xrules[110X will determine
  which  irreducible  word  is returned. In our [22Xq8[122X example we see that [22Xb^9a^-9[122X
  reduces to [22Xab[122X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xa := genfmq8[1];;  b := genfmq8[2];; [127X[104X
    [4X[25Xgap>[125X [27XA := genfmq8[3];;  B := genfmq8[4];; [127X[104X
    [4X[25Xgap>[125X [27Xw0 := b^9 * a^-9;; [127X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( w0, genfmq8, q8labs ); [127X[104X
    [4X[28Xb^9*a^-9[128X[104X
    [4X[25Xgap>[125X [27Xw1 := OnePassReduceWord( w0, rws );; [127X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( w1, genfmq8, q8labs ); [127X[104X
    [4X[28XB*b^5*a*b*a^-8[128X[104X
    [4X[25Xgap>[125X [27Xw2 := ReduceWordKB( w0, rws );; [127X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( w2, genfmq8, q8labs ); [127X[104X
    [4X[28Xa*b[128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [1X2.2-3 OnePassKB[101X
  
  [33X[1;0Y[29X[2XOnePassKB[102X( [3Xmon[103X, [3Xrules[103X ) [32X operation[133X
  
  [33X[0;0YThe  function  [10XOnePassKB[110X  implements  the  main  loop  of  the  Knuth-Bendix
  completion algorithm. Rules are compared with each other; all critical pairs
  are  calculated;  and  the  irreducible  critical  pairs are orientated with
  respect  to  the  length-lexicographical  ordering  and added to the rewrite
  system.[133X
  
  [33X[0;0YThe  function [10XShorterRule[110X gives an ordering on rules. Rules [22X(g_lg_2,id)[122X that
  identify  two generators (or one generator with the inverse of another) come
  first  in  the  ordering.  Otherwise  one precedes another if it reduces the
  length of a word by a greater amount.[133X
  
  [33X[0;0YOne  pass  of  this  procedure  for  our  [22Xq8[122X example adds [22X10[122X relators to the
  original [22X12[122X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xq1 := OnePassKB( mq8, q0 );; [127X[104X
    [4X[25Xgap>[125X [27XLength( q1 ); [127X[104X
    [4X[28X22[128X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( q1, genfmq8, q8labs ); [127X[104X
    [4X[28X[ [ a^-1, A ], [ b^-1, B ], [ A^-1, a ], [ B^-1, b ], [ a*A, id ], [128X[104X
    [4X[28X[ b*B, id ], [ A*a, id ], [ B*b, id ], [ b^2, a^2 ], [ a^3, A ], [128X[104X
    [4X[28X[ a^2*b, B ], [ a*b*a, b ], [ a*b^2, A ], [ b*a*B, A ], [ b^3, B ], [128X[104X
    [4X[28X[ a*b^2, a^3 ], [ b*a*B, a^3 ], [ b^3, a^2*b ], [ a^4, id ], [128X[104X
    [4X[28X[ a^2*b^2, id ], [ a*b*a*B, id ], [ b^4, id ] ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [1X2.2-4 RewriteReduce[101X
  
  [33X[1;0Y[29X[2XRewriteReduce[102X( [3Xmon[103X, [3Xrules[103X ) [32X operation[133X
  
  [33X[0;0YThe  function  [10XRewriteReduce[110X  will  remove  unnecessary rules from a rewrite
  system.  A  rule  is  deemed to be unnecessary if it is implied by the other
  rules,  i.e. if both sides can be reduced to the same thing by the remaining
  rules.[133X
  
  [33X[0;0YIn the example the [22X22[122X rules in [22Xq1[122X are reduced to [22X13[122X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xq2 := RewriteReduce( mq8, q1 );; [127X[104X
    [4X[25Xgap>[125X [27XLength( q2 ); [127X[104X
    [4X[28X13[128X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( q2, genfmq8, q8labs ); [127X[104X
    [4X[28X[ [ a^-1, A ], [ b^-1, B ], [ A^-1, a ], [ B^-1, b ], [ a*A, id ], [128X[104X
    [4X[28X[ b*B, id ], [ A*a, id ], [ B*b, id ], [ b^2, a^2 ], [ a^3, A ], [128X[104X
    [4X[28X[ a^2*b, B ], [ a*b*a, b ], [ b*a*B, A ] ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [1X2.2-5 KnuthBendix[101X
  
  [33X[1;0Y[29X[2XKnuthBendix[102X( [3Xmon[103X, [3Xrules[103X ) [32X operation[133X
  
  [33X[0;0YThe  function  [10XKnuthBendix[110X implements the Knuth-Bendix algorithm, attempting
  to  complete  a  rewrite  system  with  respect  to  a  length-lexicographic
  ordering.  It  calls  first  [10XOnePassKB[110X,  which  adds  rules,  and  then (for
  efficiency) [10XRewriteReduce[110X which removes any unnecessary ones. This procedure
  is  repeated  until  [10XOnePassKB[110X  adds  no  more  rules.  It  will  not always
  terminate,  but for many examples (all finite groups) it will be successful.
  The  rewrite system returned is complete, that is: it will rewrite any given
  word  in  the  free monoid to a unique irreducible; there is one irreducible
  for  each  element  of the quotient monoid; and any two elements of the free
  monoid which are in the same class will rewrite to the same irreducible.[133X
  
  [33X[0;0YThe  function [10XShorterRule[110X gives an ordering on rules. Rules [22X(g_lg_2,id)[122X that
  identify  two generators (or one generator with the inverse of another) come
  first  in  the  ordering.  Otherwise  one precedes another if it reduces the
  length of a word by a greater amount.[133X
  
  [33X[0;0YIn  the  example  the  function  [10XKnuthBendix[110X  requires  three  instances  of
  [10XOnePassKB[110X  and [10XRewriteReduce[110X to form the complete rewrite system [22Xrws[122X for the
  group shown above.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xq3 := KnuthBendix( mq8, q0 );; [127X[104X
    [4X[25Xgap>[125X [27XLength( q3 ); [127X[104X
    [4X[28X20[128X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( q3, genfmq8, q8labs ); [127X[104X
    [4X[28X[ [ a^-1, A ], [ b^-1, B ], [ A^-1, a ], [ B^-1, b ], [ a*A, id ], [128X[104X
    [4X[28X[ b*B, id ], [ A*a, id ], [ B*b, id ], [ b*a, a*B ], [ b^2, a^2 ], [128X[104X
    [4X[28X[ b*A, a*b ], [ A*b, a*B ], [ A^2, a^2 ], [ A*B, a*b ], [ B*a, a*b ], [128X[104X
    [4X[28X[ B*A, a*B ], [ B^2, a^2 ], [ a^3, A ], [ a^2*b, B ], [ a^2*B, b ] ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  
  [1X2.3 [33X[0;0YEnumerating elements[133X[101X
  
  [1X2.3-1 ElementsOfMonoidPresentation[101X
  
  [33X[1;0Y[29X[2XElementsOfMonoidPresentation[102X( [3Xmon[103X ) [32X attribute[133X
  
  [33X[0;0YThe function [10XElementsOfMonoidPresentation[110X returns a list of normal forms for
  the  elements  of the group given by the monoid presentation [10Xmon[110X. The normal
  forms  are  the  least  elements  in each equivalence class (with respect to
  length-lex  order).  When  [10Xrules[110X is a complete rewrite system for [10XG[110X the list
  returned  is  a set of normal forms for the group elements. For [10Xq8[110X this list
  is[133X
  
  
  [24X[33X[0;6Y[\; {\rm id},\; a^+,\; b^+,\; a^-,\; b^-,\; a^{+2},\; a^+b^+,\; a^+b^-\; ].[133X
  
  [124X
  
  [4X[32X  Example  [32X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xelmq8 := ElementsOfMonoidPresentation( q8 );; [127X[104X
    [4X[25Xgap>[125X [27XPrintLnUsingLabels( elmq8, genfmq8, q8labs ); [127X[104X
    [4X[28X[ id, a, b, A, B, a^2, a*b, a*B ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
