The generalized statement of the Woodward–Hoffmann rules states that "a" + "d" is odd if the reaction is allowed. Now, if "a" is even, then this implies that "d" is odd. Since "b" is odd in this case, the number of antarafacial components, "b" + "d", is even. Likewise, if "a" is odd, then "d" is even. Since "b" even in this case, the number of antarafacial components, "b" + "d", is again even. Thus, regardless of the initial assumption of parity for "a" and "b", the number of antarafacial components is even when the electron count is 4"n" + 2. Contrariwise, in the forbidden case, where "a + d" is even, we can show that, regardless of initial assumptions, "b" + "d" is odd.