Unexpected behavior of unused_vars #368

Consider:

>>> c = fmpq_mpoly_ctx.get(('x'))
>>> x = c.gen(0)
>>> one = x - x + 1
>>> one.unused_gens()
('x',)

In contrast:

>>> zero = x - x
>>> zero.unused_gens()
()

This is quite unexpected. The documentation of unused_gens specifies that "a generator is unused if it’s maximum degree is 0":

>>> one.degrees()
(0,)
>>> zero.degrees()
(-1,)

Strictly speaking, zero.unused_gens meets this specification. However, it might be more natural to change the specification to "at most 0" (or explicitly point at the special case 0 in the documentation.