  The variation of spectral subspaces for linear self-adjoint operators under
an additive bounded perturbation is considered. The objective is to estimate
the norm of the difference of two spectral projections associated with isolated
parts of the spectrum of the perturbed and unperturbed operators. Recent
results for semidefinite and general, not necessarily semidefinite,
perturbations are unified to statements that cover both types of perturbations
and, at the same time, also allow for certain perturbations that were not
covered before.
