  We present a Fukushima type decomposition in the setting of general
quasi-regular semi-Dirichlet forms. The decomposition is then employed to give
a transformation formula for martingale additive functionals. Applications of
the results to some concrete examples of semi-Dirichlet forms are given at the
end of the paper. We discuss also the uniqueness question about Doob-Meyer
decomposition on optional sets of interval type.
