  Let K be a subfield of the complex numbers, and let D be the Weyl algebra of
K-linear differential operators on K[x_1,...,x_n]. If M and N are holonomic
left D-modules we present an algorithm that computes explicit generators for
the finite dimensional vector space hom_D(M,N). This enables us to answer
algorithmically whether two given holonomic modules are isomorphic. More
generally, our algorithm can be used to get explicit generators for
ext^i_D(M,N) for any i.
