code: public Collection<String> groupValues() {
  		Set<String> set = new HashSet<String>();
  		for (FeatureI f : this) {
  			//enter in a set -- removes duplicates
  			set.add(f.group());
  		}
  
  		return set;
  	}
docs: Create a collection of all unique group ids in the list, as defined
  by the group() method of the features. For example, if the
  features are from a GFF1 file, then each group id identifies a particular gene,
  and this method returns a collection of all gene ids.
  
  @return A collection (suitable for iteration using Java's "for" loop) of all the
  group ids found in this list. The order of the values is undefined; it will not match
  the order of features in the list.