Category of partial differential rings
Cat::PartialDifferentialRing represents the
category of partial differential rings.
Cat::PartialDifferentialRing is a commutative
ring with a finite set of derivation operators
A derivation is a linear operator with product rule, i.e.
* g) equals
D_i(f) * g + f * D_i(g) for
For many partial differential rings the derivations are differentiations
with respect to some indeterminates. Thus in order to support a natural
notion it is also supposed that a method
diff(f, x) returns the partial derivation
f with respect to the indeterminate