ProductOrder
ProductOrder{n1, ..., nr} -- an optional argument of
MonomialOrder in monoids handled by the engine to
indicate that the monomial order is the product of r graded reverse lex
orders, each with n1, n2, ..., nr variables.
Caveat: If the number of degree vectors is greater than one, the
grading in each block only uses the first degree vector. This will
eventually change.
