```\n/\*\*\n \* Calculate x + y. Revert on overflow.\n \*\n \* @param x signed 64.64-bit fixed point number\n \* @param y signed 64.64-bit fixed point number\n \* @return signed 64.64-bit fixed point number\n \*/\nfunction add (int128 x, int128 y) internal pure returns (int128) {\n  int256 result = int256(x) + y;\n  require (result >= MIN\_64x64 && result <= MAX\_64x64);\n  return int128 (result);\n}\n```\n