FUNCTION:  CoxMatrix - Coxeter matrix of a root system or Coxeter group
    ,CALLING SEQUENCE:  CoxMatrix(R);,    
PARAMETERS:  R = a root system data Structure,SYNOPSIS:   
  The Coxeter matrix of R is the n x n matrix whose (i,j) entry is the
  order of s_i * s_j in W(R), where [s_1,...,s_n] denote the simple
  Reflections. Equivalently, the (i,j)-entry is m if (1-1/m)*Pi is the
  angle between the i-th and j-th simple roots of R.
  If R is a matrix whose (1,1) entry is 1, it is assumed that R is itself
  a Coxeter matrix and CoxMatrix(R) returns R. Any other matrix is assumed
  to be the Cartan matrix of a (not necessarily finite) root system.
  The Coxeter "matrix" of the empty root system is the empty list [].
  For a description of root system data Structures, see Coxeter[Structure].
EXAMPLES:   
  CoxMatrix(H4)[1,2];              yields                 5
  CoxMatrix([e1-e3,-e1-e2+2*e3]);  yields       array([[1,6],[6,1]])
  CoxMatrix(");                    yields       array([[1,6],[6,1]])
  m:=array([[2,-2,0],[-1,2,-1],[0,-2,2]]);
  CoxMatrix(m);                    yields  array([[1,4,2],[4,1,4],[2,4,1]])
    
SEE ALSO:  Base, CartanMatrix, Diagram, NameOf, Presentation, Structure):
