FUNCTION:  CoxNumber - Coxeter number of a Coxeter group
CALLING SEQUENCE:  CoxNumber(R);
PARAMETERS:  R = a root system data Structure,SYNOPSIS:   
  A Coxeter element of a Coxeter group W(R) is obtained by taking the
  product of the simple Reflections of W(R), in any order. The Coxeter
  number is the order of any Coxeter element.
  CoxNumber(R) computes the Coxeter number by using the fact that for
  irreducible R, the Coxeter number is the largest member of Degrees(R).
  The Coxeter number is also expressible as 2*NumRefl(R)/Rank(R), again
  assuming R is irreducible.
  For a description of root system data Structures, see Coxeter[Structure].
EXAMPLES:   ,  CoxNumber(E6);                  yields       12
  CoxNumber(A3^2*A1);             yields        4,    
SEE ALSO:  Degrees, NumRefl, Rank, Structure):
