  A version of Gabor expansion over a lattice of critical density is shown to
converge to an arbitrary function that belongs to domain of the oscillator
operator. This expansion is used for approximation of an arbitrary function
concentrated (in the sense of the Gabor transform) in a bounded domain D of the
phase plane. It is shown that a reasonable approximation can be done by means
of Gabor functions, whose number is equivalent to the simplectic area of D.
