 Let "A" be the number of ways of assigning the "j" output calls to the "m" middle stage switches.Let "B" be the number of these assignments which result in blocking.This is the number of cases in which the remaining "m"−"j" middle stage switches coincide with "m"−"j" of the "i" input calls, which is the number of subsets containing "m"−"j" of these calls.