  We propose a natural differential equation with respect to mass(es) to
analyze the scheme dependence problem. It is shown that the vertex functions
subtracted at an arbitrary Euclidean momentum (MOM) do not satisfy such
differential equations, as extra unphysical mass dependence is introduced which
is shown to lead to the violation of the canonical form of the Slavnov-Taylor
identities, a notorious fact with MOM schemes. By the way, the traditional
advantage of MOM schemes in decoupling issue is shown to be lost in the context
of Callan-Symanzik equations.
