  We derive a path-integral Schwinger-Keldysh approach for quantum spin
systems. This is achieved by means of a semionic representation of spins as
fermions with imaginary chemical potential. The major simplifying feature in
comparison with other representations (Holstein-Primakoff, Dyson-Maleev, slave
bosons/fermions etc) is that the local constraint is taken into account
exactly. As a result, the standard diagram technique with usual Feynman codex
is constructed. We illustrate the application of this technique for the N\'eel
and spin-liquid states of the AFM Heisenberg model.
