OpenSSL::PKey::EC#group=

static VALUE
ossl_ec_key_set_group(VALUE self, VALUE group_v)
{
#if OSSL_OPENSSL_PREREQ(3, 0, 0)
    rb_raise(ePKeyError, "pkeys are immutable on OpenSSL 3.0");
#else
    EC_KEY *ec;
    EC_GROUP *group;

    GetEC(self, ec);
    GetECGroup(group_v, group);

    if (EC_KEY_set_group(ec, group) != 1)
        ossl_raise(eECError, "EC_KEY_set_group");

    return group_v;
#endif
}