FUNCTION:  LengthGF - length generating function for a Coxeter group,    
CALLING SEQUENCE:  LengthGF(R);,                   LengthGF(R,z);,    
PARAMETERS:  R = a root system data Structure
             z = a variable or expression (optional),SYNOPSIS:   
  The length of an element w in W(R) is the minimum length of an expression
  for w as a product of simple Reflections. The length generating function
  (or Poincare polynomial) P(q) for W(R) is the sum of q^length(w) for all
  w in W(R).
  LengthGF(R,z) returns the length generating function for W(R) using the
  formula P(z) = product((1-z^d[i])/(1-z),i=1..Rank(R)), where d[1],d[2],...
  are the Degrees of the basic polynomial invariants for W(R). If the second
  argument is omitted, q is used by default.
  For a description of root system data Structures, see Coxeter[Structure].
EXAMPLES:   
  LengthGF(A2,z);                  yields        z^3+2*z^2+2*z+1
  LengthGF(B3)/LengthGF(A2);
  normal(");                        yields   q^6+q^5+q^4+2*q^3+q^2+q+1,    
