static void verifyFunctionPairOnInput(O forward, O backward, O input) {
  O intermediate, output;
  if (!eq(input, output = callF(backward, intermediate = callF(forward, input))))
    fail("Function is not a pair: input=" + sfu(input) + ", output=" + sfu(output) + ", f=" + forward + ", g=" + backward + ", intermediate=" + sfu(intermediate));
}