auto* erf_user_user = GetUserIfOnlyOne(erf_user);
    if (!erf_user_user) return failure();

    // `mul 1/sqrt(2)`
    auto* erf_input = GetInputOpWithOneUse(op, 0);
    if (!erf_input) return failure();

    // `mul 0.5`
    auto* erf_user_user_input = GetInputOpWithOneUse(erf_user_user, 0);
    if (!erf_user_user_input) return failure();

    // Check `mul 0.5` and `mul 1/sqrt(2)` refer to the same input.