Function synonym

src/parse.js:459–464

* Generate a function that curries a destination key for a flattener * @private * @param {string} key the eventual destination key * @returns {Function} a flattener that remembers that key

(key)

 * @returns {Function} a flattener that remembers that key
 */
function synonym(key) {
  return function (result, tag) {
    const fun = flatteners[key];
    fun.apply(null, [result, tag, key].slice(0, fun.length));
  };
}

/**
 * Treat the existence of a tag as a sign to mark `key` as true in the result