GROEBOPT _ _ _ _ _ _ _ _ _ _ _ _ switch
If groebopt is set ON, the sequence of variables is optimized with respect to execution speed of groebner calculations; note that the final list of variables is available in gvarslast. By default groebopt is off, conserving the original variable sequence.
An explicitly declared dependency using the depend declaration supersedes the variable optimization.
guarantees that a will be placed in front of x and y.