void overload_add_eq(pTHX_ SV * a, SV * b, SV * third) {
     dXSARGS;
     PERL_UNUSED_ARG(third);
     mpz_add_ui(*(INT2PTR(mpz_t *, SvIVX(SvRV(a)))), *(INT2PTR(mpz_t *, SvIVX(SvRV(a)))), SvUV(b));
     XSRETURN(1);
}