D-modules over Rational Function Field

[mahrud]

@antonleykin @mikestillman what are the changes needed to support working with D-modules over rational function fields? I think this is a theoretically sensible ring to work in (derivatives of rational functions are rational functions), and for instance Theorem 1.4.22 in the Grobner Deformations book discusses Grobner bases over rational function fields, so there are applications for this as well. Can you think of any particular dangers to be aware of?