  The four dimensional SU(2) WZW model coupled to elecromagnetism is treated as
a constraint system in the context of the BFV approach. We show that the
Darboux's transformations which are used to diagonalize the canonical one-form
in the Faddeev-Jackiw formalism, transform the fields of the model into BRST
invariant ones. The same analysis is also carried out in the case of spinor
electrodynamics.
