  The relative error of cloning of quantum states with arbitrary prior
probabilities is considered. It is assumed that the ancilla may contain some a
priori information about the input state to be cloned. The lower bound on the
relative error for general cloning scenario is derived. Both the case of
two-state set and case of multi-state set are analyzed in details. The treated
figure of merit is compared with other optimality criteria. The quantum circuit
for optimal cloning of a pair of pure states is constructed.
