  We glue together two branched spheres by sewing of two Ramond (dual)
two-fermion string vertices and present a rigorous analytic derivation of the
closed expression for the four-fermion string vertex. This method treats all
oscillator levels collectively and the obtained answer verifies that the closed
form of the four vertex previously argued for on the basis of explicit results
restricted to the first two oscillator levels is the correct one.
