has part has part has_part has_sequence_feature sequence feature set qualified sequence feature or collection qualified sequence feature qualified sequence feature set A set of qualified seqeunce features. 'Sets' are used to represent entities that are typically collections of more than one member. But we allow for sets that contain 0 members (an 'empty' set) or 1 member (a 'singleton' or 'unit' set), consistent with the concept of 'matehmatical sets'.