 A non-empty subset "T" of formula_3 is a triangular set, 
if the polynomials in "T" are non-constant and have distinct main variables.Hence, a triangular set is finite, and has cardinality at most "n".Let T = {t1, ..., ts} be a triangular set such that 
"mvar"(t1) < ... < "mvar"(ts), 
formula_8 be the initial of "t"i and "h" be the product of hi's.