code: public FeatureList selectByUserData(String key) {
  		FeatureList list = new FeatureList();
  		for (FeatureI f : this) {
  			if (f.userData().containsKey(key)) {
  				list.add(f);
  			}
  		}
  		return list;
  	}
docs: Create a list of all features that include the specified key in their userMap().
  
  @param key The key to consider.
  @return A list of features that include the key.