// @noImplicitAny: true
/**
 * @constructor
 * @template {string} K
 * @template V
 */
// function Multimap() {
var Multimap = function() {
    /** @type {Object<string, V>} TODO: Remove the prototype from the fresh object */
    this._map = {};
};

Multimap.prototype = {
  /**
   * @param {K} key
   * @param {V} value
   */
  set: function(key, value) {
    var set = this._map.get(key);
    if (!set) {
      set = new Set();
      this._map.set(key, set);
    }
    set.add(value);
  }
}