LUA_API void  lua_arith (lua_State *L, int op) {
  StkId o1;  /* 1st operand */
  StkId o2;  /* 2nd operand */
  lua_lock(L);
  if (op != LUA_OPUNM) /* all other operations expect two operands */
    api_checknelems(L, 2);
  else {  /* for unary minus, add fake 2nd operand */
    api_checknelems(L, 1);
    setobjs2s(L, L->top, L->top - 1);
    L->top++;
  }
  o1 = L->top - 2;
  o2 = L->top - 1;
  if (ttisnumber(o1) && ttisnumber(o2)) {
    changenvalue(o1, luaO_arith(op, nvalue(o1), nvalue(o2)));
  }
  else
    luaV_arith(L, o1, o1, o2, cast(TMS, op - LUA_OPADD + TM_ADD));
  L->top--;
  lua_unlock(L);
}