Woodward–Hoffmann rules
Finally, to complete the argument, and show that this new criterion is truly equivalent to the original criterion, one needs to argue the converse statements as well, namely, that the number of antarafacial components "b" + "d" and the electron count (4"n" + 2 or 4"n") implies the parity of "a" + "d" that is given by the Woodward–Hoffmann rules (odd for allowed, even for forbidden). Another round of (somewhat tedious) case analyses will easily show this to be the case.