Efficient Forney functions for decoding AG codes

Using a Forney formula to solve for the error magnitudes in decoding AG cod es requires producing functions sigma(P), which are 0 at all but one point P of the variety of the error-locator ideal. The best such function is prod uced here in a reasonably efficient way from a lex Grobner basis. This lex basis is, in turn, produced efficiently from a weighted, grevlex basis by u sing the FGLM algorithm. These two steps essentially complete the efficient decoding scheme based on a Forney formula started in the author's previous work.