DifferenceMod - computes indefinite sums of expressions containing unknown functions and their shifts

Calling Sequences
 DifferenceMod(E,n)

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


Remarks
-The non-locality (difference between the highest and lowest shift) of the result is minimized.
-The form of the summand 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.