  The definition of Q-conditional symmetry for one PDE is correctly generalized
to a special case of systems of PDEs and involutive families of operators. The
notion of equivalence of Q-conditional symmetries under a group of local
transformation is introduced. Using this notion, all possible single
Q-conditional symmetry operators are classified for the n-dimensional (n >= 2)
linear heat equation and for the Euler equations describing the motion of an
incompressible ideal fluid.
