  We prove that any potential symmetry of a system of evolution equations
reduces to a Lie symmetry through a nonlocal transformation of variables. Based
on this fact is our method of group classification of potential symmetries of
systems of evolution equations having non-trivial Lie symmetry. Next, we modify
the above method to generate more general nonlocal symmetries, which yields a
purely algebraic approach to classifying nonlocal symmetries of evolution type
systems. Several examples are considered.
