  Closed string diagrams are derived from cubic open string field theory using
a gauge fixed kinetic operator. The basic idea is to use a string propagator
that does not generate a boundary to the world sheet. Using this propagator and
the closed string vertex, the moduli space of closed string surfaces is
covered, so closed string scattering amplitudes should be reproduced. This
kinetic operator could be a gauge fixed form of the string field theory action
around the closed string vacuum.
