FUNCTION:  Rho    - half the sum of the positive roots
           CoRho - half the sum of the positive co-roots,    
CALLING SEQUENCE:  Rho(R);,                   CoRho(R);,    
PARAMETERS:  R = a root system data Structure,SYNOPSIS:   
  Rho(R) returns half the sum of the positive roots of the root system R,
  or equivalently, the sum of the fundamental Weights of R.
  CoRho(R) returns half the sum of the positive co-roots, or equivalently,
  the unique vector v in the span of Base(R) such that Iprod(r,v)=1 for
  each simple root r.
  For a description of root system data Structures, see Coxeter[Structure].
EXAMPLES:   ,  Rho(C3);                yields        e1+2*e2+3*e3
  2*CoRho(C3);           yields        e1+3*e2+5*e3,    
SEE ALSO:  Weights, Coxeter[Base], Coxeter[PosRoots], Coxeter[Structure]):
