OpenSSL::PKey::EC#private_key=

static VALUE ossl_ec_key_set_private_key(VALUE self, VALUE private_key)
{
#ifdef OSSL_HAVE_IMMUTABLE_PKEY
    rb_raise(ePKeyError, "pkeys are immutable on OpenSSL 3.0");
#else
    EC_KEY *ec;
    BIGNUM *bn = NULL;

    GetEC(self, ec);
    if (!NIL_P(private_key))
        bn = GetBNPtr(private_key);

    switch (EC_KEY_set_private_key(ec, bn)) {
      case 1:
        break;
      case 0:
        if (bn == NULL)
            break;
        /* fallthrough */
      default:
        ossl_raise(ePKeyError, "EC_KEY_set_private_key");
    }

    return private_key;
#endif
}