code: function make(x, options = {}) {
    var {debug = DEBUG_ALL} = options;
    var u;
  
    debug = DEBUG && debug;
  
    if (x === undefined) u = makeUndefined();
    else if (x === null) u = makeNull();
    else {
      u = Object(x);
      if (!is(u)) {
        add(u, debug);
        defineProperty(u, 'change', { value: change });
      }
    }
    if (debug) console.debug(...channel.debug("Created upwardable", u._upwardableId, "from", x));
    return u;
  }
docs: Make a new upwardable. Register it, and add a `change` method which notifies when it is to be replaced.