FUNCTION:  Presentation - generators and relations for a Coxeter group,    
                   Presentation(R,export);,    
PARAMETERS:  R = a root system data Structure,SYNOPSIS:   
  Presentation(R) returns the Coxeter Presentation of W(R). The output is
  a "grelgroup" (see group[grelgroup]). More specifically, the result
  returned is an unevaluated procedure call
               grelgroup({s1,s2,...}, {<rel1>, <rel2>,...})
  where s1,s2,... are names for the simple Reflections, and each expression
  <rel_i> is a relation (i.e., a sequence of generators and inverses of
  generators whose product in W(R) is the identity element).
  If 'export' is specified as a second argument, then the output is a
  sequence of commands for the GAP language that define W(R) by its
  Coxeter Presentation. For more information about GAP, see
  http://www.ccs.neu.edu/mirrors/GAP/.
  For a description of root system data Structures, see Coxeter[Structure].
EXAMPLES:   ,  Presentation(A2);                     yields  
                   grelgroup({s1,s2},{[s1,s1],[s2,s2],[s1,s2,s1,s2,s1,s2]})
  #Is W(H3) generated by s1 and s2*s3 ?
  g:=Presentation(H3); with(group,cosets);
  sg:=subgrel({t1=[s1],t2=[s2,s3]},g);
  cosets(sg);                           yields         {[]}
SEE ALSO:  Base, CoxMatrix, PermRep, Structure, group[grelgroup]):
