  For a given ideal I in K[x_1,...,x_n,y_1,...,y_m] in a polynomial ring with
n+m variables, we want to find all elements that can be written as f-g for some
f in K[x_1,...,x_n] and some g in K[y_1,...,y_m], i.e., all elements of I that
contain no term involving at the same time one of the x_1,...,x_n and one of
the y_1,...,y_m. For principal ideals and for ideals of dimension zero, we give
a algorithms that compute all these polynomials in a finite number of steps.
