DerivativeMod - computes an expression modulo total derivatives

Calling Sequences
 DerivativeMod(E,x)

Parameters
 E - an expression containing unknown and known functions of the integration variable x.
 x - the independent variable.


Remarks
-The differential order of the result minimized.
-The form of the expression can effect the procedure for rational expressions containing non-monomial terms in the      denominator.  In such cases the result is always corect, though it may not be optimal.