enum_monomial returns an incorrect monomia

[ksuenaga]

Calling enum_monomial with the following parameters:

degree:2
type environment : x0:ty0^1, x2:ty2^1, x1:ty2^{-1}*ty0^1 (ctenv is empty)
typ:ty0 * 2

returns the following monomials:

x0^2, x0, x1^2

However, x1^2 have the type ty2^{-2}*ty0^2, which has an incorrect type.