  The formalism of Ashtekar and Magnon \cite{AshtekarMagnon:1975} for the
definition of particles in quantum field theory in curved spacetime is further
developed. The relation between basic objects of this formalism (e.g., the
complex structure) and different Green functions is found. It allows one to
derive composition laws for Green functions. The relation of two definitions of
particles is reformulated in the formalism and the base-independent Bogoljubov
transformation is expressed using quantities which are derivable directly from
the ``in-out'' Green function.
