  
  [1X3 Rational languages[0X
  
  Rational   languages   are   conveniently   represented   through   rational
  expressions. These are finite expressions involving letters of the alphabet;
  [10Xconcatenation[0m,  corresponding to the [13Xproduct[0m; the symbol [10XU[0m, corresponding to
  the [13Xunion[0m; and the symbol [10X*[0m, corresponding to the Kleene's star.
  
  
  [1X3.1 Rational Expressions[0X
  
  The  expressions [10X@[0m and [10X"empty\_set"[0m are used to represent the empty word and
  the empty set respectively.
  
  [1X3.1-1 RationalExpression[0m
  
  [2X> RationalExpression( [0X[3Xexpr[, alph][0X[2X ) _______________________________[0Xfunction
  
  A  rational expression can be created using the function [10XRationalExpression[0m.
  [3Xexpr[0m is a string representing the desired expression literally and [3Xalph[0m (may
  or  may  not  be  present) is the alphabet of the expression. Of course [3Xalph[0m
  must  not  contain  the symbols '@', '(', ')', '*' nor 'U'. When [3Xalph[0m is not
  present,  the  alphabet  of  the  rational  expression is the set of symbols
  (other  than '"', etc...) occurring in the expression. (The alphabet is then
  ordered with the alphabetical order.)
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> RationalExpression("abUc");[0X
    [4XabUc[0X
    [4Xgap> RationalExpression("ab*Uc");[0X
    [4Xab*Uc[0X
    [4Xgap> RationalExpression("001U1*");[0X
    [4X001U1*[0X
    [4Xgap> RationalExpression("001U1*","012");[0X
    [4X001U1*[0X
  [4X------------------------------------------------------------------[0X
  
  [1X3.1-2 RatExpOnnLetters[0m
  
  [2X> RatExpOnnLetters( [0X[3Xn, operation, list[0X[2X ) ___________________________[0Xfunction
  
  This is another way to construct a rational expression over an alphabet. The
  user  may specify the alphabet or just give the number n of letters (in this
  case  the  alphabet  {a,b,c,...} is considered). [3Xoperation[0m is the name of an
  operation,  the  possibilities being: [10Xproduct[0m, [10Xunion[0m or [10Xstar[0m. [3Xlist[0m is a list
  of rational expressions, a rational expression in the case of ``star'', or a
  list  consisting  of  an  integer  when  the rational expression is a single
  letter.  The empty list [10X[ ][0m and [10Xempty\_set[0m are other possibilities for [10Xlist[0m.
  An example follows
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> RatExpOnnLetters(2,"star",RatExpOnnLetters(2,"product",[0X
    [4X> [RatExpOnnLetters(2,[],[1]),RatExpOnnLetters(2,"union",[0X
    [4X> [RatExpOnnLetters(2,[],[1]),RatExpOnnLetters(2,[],[2])])]));      [0X
    [4X(a(aUb))*[0X
  [4X------------------------------------------------------------------[0X
  
  The empty word and the empty set are the rational expressions:
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> RatExpOnnLetters(2,[],[]);         [0X
    [4X@[0X
    [4Xgap> RatExpOnnLetters(2,[],"empty_set");[0X
    [4Xempty_set[0X
  [4X------------------------------------------------------------------[0X
  
  [1X3.1-3 RandomRatExp[0m
  
  [2X> RandomRatExp( [0X[3Xarg[0X[2X ) ______________________________________________[0Xfunction
  
  Given  the number of symbols of the alphabet and (possibly) a factor m which
  is  intended to increase the randomality of the expression, returns a pseudo
  random rational expression over that alphabet.
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> RandomRatExp(2);[0X
    [4Xb*(@Ua)[0X
    [4Xgap> RandomRatExp("01");[0X
    [4Xempty_set[0X
    [4Xgap> RandomRatExp("01");[0X
    [4X(0U1)*[0X
    [4Xgap> RandomRatExp("01",7);[0X
    [4X0*1(@U0U1)[0X
  [4X------------------------------------------------------------------[0X
  
  [1X3.1-4 SizeRatExp[0m
  
  [2X> SizeRatExp( [0X[3Xr[0X[2X ) __________________________________________________[0Xfunction
  
  Returns  the  size,  i.e.  the  number  of  symbols  of the alphabet, of the
  rational expression [3Xr[0m.
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> a:=RationalExpression("0*1(@U0U1)");[0X
    [4X0*1(@U0U1)[0X
    [4Xgap> SizeRatExp(a);[0X
    [4X5[0X
  [4X------------------------------------------------------------------[0X
  
  [1X3.1-5 IsRationalExpression[0m
  
  [2X> IsRationalExpression( [0X[3Xr[0X[2X ) ________________________________________[0Xfunction
  
  Tests whether an object is a rational expression.
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> IsRatExpOnnLettersObj(r3) and IsRationalExpressionRep(r3);[0X
    [4Xtrue[0X
    [4Xgap> IsRationalExpression(r1);[0X
    [4Xtrue[0X
  [4X------------------------------------------------------------------[0X
  
  [1X3.1-6 AlphabetOfRatExp[0m
  
  [2X> AlphabetOfRatExp( [0X[3XR[0X[2X ) ____________________________________________[0Xfunction
  
  Returns the number of symbols in the alphabet of the rational expression [10XR[0m.
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> r:=RandomRatExp(2);[0X
    [4Xa*(ba*U@)[0X
    [4Xgap> AlphabetOfRatExp(r);[0X
    [4X2[0X
    [4Xgap> r:=RandomRatExp("01");[0X
    [4X1*(01*U@)[0X
    [4Xgap> AlphabetOfRatExp(r);[0X
    [4X2[0X
    [4Xgap> a:=RandomTransformation(3);;[0X
    [4Xgap> b:=RandomTransformation(3);;[0X
    [4Xgap> r:=RandomRatExp([a,b]);[0X
    [4X(Transformation( [ 1, 1, 3 ] )UTransformation( [ 1, 1, 2 ] ))*[0X
    [4Xgap> AlphabetOfRatExp(r);[0X
    [4X[ Transformation( [ 1, 1, 3 ] ), Transformation( [ 1, 1, 2 ] ) ][0X
  [4X------------------------------------------------------------------[0X
  
  [1X3.1-7 AlphabetOfRatExpAsList[0m
  
  [2X> AlphabetOfRatExpAsList( [0X[3XR[0X[2X ) ______________________________________[0Xfunction
  
  Returns  the  alphabet of the rational expression [10XR[0m always as a list. If the
  alphabet  of  the  rational  expression is given by means of an integer less
  than  27  it  returns  the  list [10X"abcd...."[0m, otherwise returns [10X[ "a1", "a2",
  "a3", "a4", ... ][0m.
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> r:=RandomRatExp(2);[0X
    [4X(aUb)((aUb)(bU@)U@)U@[0X
    [4Xgap> AlphabetOfRatExpAsList(r);[0X
    [4X"ab"[0X
    [4Xgap> r:=RandomRatExp("01");[0X
    [4X1*(0U@)[0X
    [4Xgap> AlphabetOfRatExpAsList(r);[0X
    [4X"01"[0X
    [4Xgap> r:=RandomRatExp(30);;[0X
    [4Xgap> AlphabetOfRatExpAsList(r);[0X
    [4X[ "a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", "a10", "a11", [0X
    [4X"a12", "a13", "a14", "a15", "a16", "a17", "a18", "a19", "a20", "a21", [0X
    [4X"a22", "a23", "a24", "a25", "a26", "a27", "a28", "a29", "a30" ][0X
  [4X------------------------------------------------------------------[0X
  
  [1X3.1-8 CopyRatExp[0m
  
  [2X> CopyRatExp( [0X[3XR[0X[2X ) __________________________________________________[0Xfunction
  
  Returns a new rational expression, which is a copy of [10XR[0m.
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> r:=RandomRatExp(2);[0X
    [4Xa*(bU@)[0X
    [4Xgap> CopyRatExp(r);[0X
    [4Xa*(bU@)[0X
  [4X------------------------------------------------------------------[0X
  
  
  [1X3.2 Comparison of rational expressions[0X
  
  The way two rational expressions [10Xr1[0m and [10Xr2[0m are compared through the operator
  is the following: the empty set is lesser than everything else; if r1 and r2
  are  letters, then the lesser is taken from the order in the alphabet; if r1
  is  a  letter  an  r2 a product, union or star, then r1 is lesser than r2; a
  star  expression  is  considered  to  be  lesser  than  a  product  or union
  expression  and  a  product  lesser  than  a  union;  to  compare  two  star
  expressions  we  compare  the  expressions  under  the stars; to compare two
  product   or  union  expressions  we  compare  the  subexpressions  of  each
  expression from left to right;
  
  
  [1X3.3 Operations with rational languages[0X
  
  Only operations with rational languages over the same alphabet are allowed.
  
  We  may compute expressions for the [10Xproduct[0m, [10Xunion[0m and [10Xstar[0m (i.e., submonoid
  generated   by)  of  rational  sets.  In  some  cases,  simpler  expressions
  representing  the  same set are returned. Note that that two simplifications
  are  always  made,  namely, and . Of course, these operations may be used to
  construct  more  complex  expressions.  For rational expressions we have the
  functions  [10XUnionRatExp[0m,  [10XProductRatExp[0m, [10XStarRatExp[0m, that return respectively
  rational expressions for the [13Xunion[0m and [13Xproduct[0m of the languages given by the
  rational  expressions  [10Xr[0m  and  [10Xs[0m  and  the [10Xstar[0m of the language given by the
  rational expression [10Xr[0m.
  
  [1X3.3-1 UnionRatExp[0m
  
  [2X> UnionRatExp( [0X[3Xr, s[0X[2X ) ______________________________________________[0Xfunction
  [1X3.3-2 ProductRatExp[0m
  
  [2X> ProductRatExp( [0X[3Xr, s[0X[2X ) ____________________________________________[0Xfunction
  [1X3.3-3  StarRatExp[0m
  
  [2X>  StarRatExp( [0X[3Xr[0X[2X ) _________________________________________________[0Xfunction
  
  The expression [10X(a(aUb))*[0m may be produced in the following way
  
  [4X---------------------------  Example  ----------------------------[0X
    [4Xgap> r1 := RatExpOnnLetters(2,[],[1]); [0X
    [4Xa[0X
    [4Xgap> r2 := RatExpOnnLetters(2,[],[2]);[0X
    [4Xb[0X
    [4Xgap> r3 := UnionRatExp(r1,r2);[0X
    [4XaUb[0X
    [4Xgap> r4 := ProductRatExp(r1,r3);[0X
    [4Xa(aUb)[0X
    [4Xgap> r5 := StarRatExp(r4);[0X
    [4X(a(aUb))*[0X
  [4X------------------------------------------------------------------[0X
  
