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

Caveat

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.

For the programmer

The object GroupLex is .