code: function Expand(mapper, sink, source) {
  	this.error = null;
  	this.items = [];
  	this.mapper = mapper || identity;
  	this.selectPending = false;
  	this.sink = sink;
  	this.sink.source = this;
  	this.state = Asyncplify.states.RUNNING;
      this.source = null;
  	this.subscribables = [];
  	this.value = undefined;
  
      source._subscribe(this);
  }
docs: TODO Add isSubscribing to expand TODO Better implement pause