FUNCTION:  PosRoots - positive roots of a root system,    
CALLING SEQUENCE:  PosRoots(R);,    
PARAMETERS:  R = a root system data Structure,SYNOPSIS:   
  PosRoots(R) returns a list of all positive roots of R; i.e., all roots
  that are nonnegative linear combinations of the simple roots.
  If any floating-point coordinates occur in Base(R), then the result is
  only approximate in the sense that roots r1 and r2 whose squared distance
  is at most epsilon ( = 0.001, by default) are considered to be equal.,   

  For a description of root system data Structures, see Coxeter[Structure].
EXAMPLES:   
  PosRoots(B2);                      yields     [e1, e2-e1, e2, e2+e1]
  r:=HighestRoot(B2); S:=Base(B2);
  S:=[S[1],-r]; PosRoots(S);         yields    [e1, -e2-e1, -e2, e1-e2]
    ,SEE ALSO:  Base, HighestRoot, NumRefl, Orbit, Structure, Weyl[Rho]):
