  We present a unified framework for equilibrium and nonequilibrium many-body
perturbation theory. The most general nonequilibrium many-body theory valid for
general initial states is based on a time-contour originally introduced by
Konstantinov and Perel'. The various other well-known formalisms of Keldysh,
Matsubara and the zero-temperature formalism are then derived as special cases
that arise under different assumptions. We further present a single simple
proof of Wick's theorem that is at the same time valid in all these flavors of
many-body theory. It arises simply as a solution of the equations of the
Martin-Schwinger hierarchy for the noninteracting many-particle Green's
function with appropriate boundary conditions. We further discuss a generalized
Wick theorem for general initial states on the Keldysh contour and derive how
the formalisms based on the Keldysh and Konstantinov-Perel'-contours are
related for the case of general initial states.
