Function dc#Identifier (? -> string)  to specify a reference that is unambiguous within a given context; SHOULD NOT BE USED IN WEBKB
  supertype:  name (pm)
  subtype:  identifier (entity -> Literal)  a relation between an individual of a concept and the identifier of that concept; the identifier is a concrete data type, usually non_meaningful from the lexical viewpoint; useful to map many database relationships