var a = [0, 0, 0];

    // Accept input from user
    function modify(i, v) {
      a[i] = v;
      // ...
    }

    // Trigger an event when two values are equal
    function waitUntil(i, j) {
      return new Promise(function(res, rej) {
        // ?
      });
    }

  var EventEmitter = require('events').EventEmitter;
  var Promise = require('promise');

  function Answer() {
    var values = [];
    var emitters = [];

    this.modify = function(index, value) {
        values[index] = value;
        if (emitters[index]) {
            emitters[index].emit('modified');
        }
    };

    this.waitUntilEqual = function(index1, index2) {
        return new Promise(function(resolve) {
            if (!emitters[index1]) {
                emitters[index1] = new EventEmitter();
            }
            if (!emitters[index2]) {
                emitters[index2] = new EventEmitter();
            }
            function compare() {
                if (values[index1] === values[index2]) {
                    resolve();
                }
            }
            emitters[index1].on('modified', compare);
            emitters[index2].on('modified', compare);
        });
    };
  }

  var check = new Answer();
  check.waitUntilEqual(0, 1).then(function () {   console.log('equal'); });
  check.modify(0, 'x');
  check.modify(1, 'x');