  Consequent application of the instantaneous approximation to both the
interaction and all propagators of the bound-state constituents allows us to
forge, within the framework of the Bethe-Salpeter formalism for the description
of bound states, an instantaneous form of the Bethe-Salpeter equation with
exact (i.e., full) propagators of the bound-state constituents. This
instantaneous equation generalizes the well-known Salpeter equation the
derivation of which needs the additional assumption of free propagation of the
bound-state constituents.
