FUNCTION:  HighestRoot - highest root in an irreducible root system,    
CALLING SEQUENCE:  HighestRoot(R);
PARAMETERS:  R = a root system data Structure,SYNOPSIS:   
  A vector v is dominant if it belongs to the closure of the fundamental
  chamber; i.e., Iprod(r,v) >= 0 for all simple roots r of R.
  HighestRoot(R) returns a dominant root of R of maximum length. If R
  is crystallographic and irreducible, there is a unique such root, the
  so-called "highest root".
  For a description of root system data Structures, see Coxeter[Structure].
EXAMPLES:   
  HighestRoot(C4);                   yields          2*e4
  HighestRoot(I2[5]);
  evalf(",3);                         yields   .309*e1 + .951*e2,    
SEE ALSO:  Base, PosRoots, Vec2FC, Structure):
