[v1 ... vn] [w1 ... wn] m ip1 [f1 ... fs]
<poly> v1 ... vn ; <integer> w1 ... wn m
<poly> f1 ... fs
Example: [(x,y) ring_of_differential_operators 0] define_ring
[(Dx). (Dy).] [2 1] 3 ip1 :: [(2 Dx Dy). (Dy^3).]
Returns Dx^p Dy^q such that 2 p + 1 q = 3.