SV * overload_add_eq(pTHX_ SV * p, SV * a, SV * b) {
     mpz_add_ui(*(INT2PTR(mpz_t *, SvIVX(SvRV(p)))), *(INT2PTR(mpz_t *, SvIVX(SvRV(p)))), SvUV(a));
     SvREFCNT_inc(p);
     return p;
}