if (expr[i].type == "op") {

            if (expr[i].specifier == "+" ||
                expr[i].specifier == "-" ||
                expr[i].specifier == "*" ||
                expr[i].specifier == "/")
            {
                if ((expr[i - 1].type == "int" ||
                    expr[i - 1].type == "float" ||
                    expr[i - 1].type == "string" ||
                    expr[i - 1].type == "var_int" ||
                    expr[i - 1].type == "var_float" ||
                    expr[i - 1].type == "var_string" ||
                    expr[i - 1].type == "func" ||
                    expr[i - 1].type == "user_func" ||
                    expr[i - 1].specifier == ")")
                    &&
                    (expr[i + 1].type == "int" ||
                        expr[i + 1].type == "float" ||
                        expr[i + 1].type == "string" ||
                        expr[i + 1].type == "var_int" ||
                        expr[i + 1].type == "var_float" ||
                        expr[i + 1].type == "var_string" ||
                        expr[i + 1].type == "func" ||
                        expr[i + 1].type == "user_func" ||
                        expr[i + 1].specifier == "("))
                {
                }
                else {
                    std::cout << ">>> SYNTAX ERROR: invalid operator placement. (Operator)" << std::endl;
                    return false;
                }
            }
        }