GroupLex -- defines a ring where some variables are inverted

Description

MonomialOrder => GroupLex => n inverts the first n variables in the polynomial ring. In the following example, a^-1 is in the ring, but c^-1 is not.

i1 : R = QQ[a..d, MonomialOrder=> GroupLex=>2];

i2 : a^-1

      -1
o2 = a

o2 : R

i3 : try c^(-1) else "failed"

o3 = failed

The element a/b is in the fraction ring, while a*b^(-1) belongs to R.

Currently, one cannot compute Gröbner bases in this ring.

The object GroupLex is a symbol.