Function arith_add

third-party/lua-5.5.0/src/lstrlib.c:299–301

static int arith_add (lua_State *L) {
  return arith(L, LUA_OPADD, "__add");
}

static int arith_sub (lua_State *L) {
  return arith(L, LUA_OPSUB, "__sub");