arguments: k:field M:monoid a:intarray >> ring
Create the ring of differential operators (the Weyl algebra), whose underlying commutative polynomial ring is k[M], and where the differential operators are given by 'a'. (E.g. a = {0,2,1,3} associates variable 2 to be the differential operator corresponding to the variable 0, var 3 is the diff op corresponding to var 1, etc. A negative value signifies that the corresponding diff op, or commutative variable, is not present.