docs: function (computation) {
      if (! computation) {
        if (! Deps.active)
          return false;
  
        computation = Deps.currentComputation;
      }
      var self = this;
      var id = computation._id;
      if (! (id in self._dependentsById)) {
        self._dependentsById[id] = computation;
        computation.onInvalidate(function () {
          delete self._dependentsById[id];
        });
        return true;
      }
      return false;
    }