code: func Single(iter Enumerable) (retval interface{}, err error) {
  	done := make(chan struct{})
  	defer close(done)
  
  	err = errNoElements
  
  	firstPass := true
  	for entry := range iter.Enumerate(done) {
  		if firstPass {
  			retval = entry
  			err = nil
  		} else {
  			retval = nil
  			err = errMultipleElements
  			break
  		}
  		firstPass = false
  	}
  	return
  }
docs: // Single retreives the only element from a list, or returns nil and an error.