FUNCTION:  Rank - Rank of a root system or Coxeter group,    
CALLING SEQUENCE:  Rank(R);,    
PARAMETERS:  R = a root system data Structure,SYNOPSIS:   
  Rank(R) returns the Rank of the root system R; i.e., the dimension of the
  vector space spanned by the roots of R, or equivalently, the number of
  simple Reflections in W(R).
  For a description of root system data Structures, see Coxeter[Structure].
EXAMPLES:   ,  Rank(I2[7]);                        yields     2
  Rank(A3^2*B3*D10);                  yields     19
  m:=CartanMatrix(C4); Rank(m);      yields     4,    
SEE ALSO:  Base, Diagram, Structure):
