FUNCTION: LieTools/gActionSg
USAGE: gActionSg(X,gpoly); gActionSg(i,gpoly);
SYNOPSIS: gActionSg(X,gpoly) computes the action of an element X in g on 
  a polynomial gpoly in S(g) (here X should be a linear expression in the
  basis elements g[i]).
  Alternatively, if the first argument is an integer i, then gActionSg(i,gpoly)
  returns the action of the basis element g[i] on the polynomial gpoly.
CAVEATS: requires a previously initialized global Lie algebra environment.
  (See LieTools[gSetup].)
EXAMPLE: 
> LieTools[gSetup](G,2):
Setting up Gtype = G   rnk = 2
> LieTools[gActionSg](g[1],g[14]^2);

                     -2 g[14] g[7] - 4 g[14] g[8]

> LieTools[gActionSg](1,g[14]^2);

                     -2 g[14] g[7] - 4 g[14] g[8]

SEE ALSO: LieTools[Dg]
