code: function maybeReadMore(stream, state) {
  	  if (!state.readingMore) {
  	    state.readingMore = true;
  	    nextTick(maybeReadMore_, stream, state);
  	  }
  	}
docs: at this point, the user has presumably seen the 'readable' event, and called read() to consume some data.  that may have triggered in turn another _read(n) call, in which case reading = true if it's in progress. However, if we're not ended, or reading, and the length < hwm, then go ahead and try to read some more preemptively.