Function dl#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
  inverse:  identifier_of
  supertype:  extrinsic_relation__extrinsicrelation (?,Literal)  support for relations ranging on concrete data types
     supertype:  relation_to_string (*,string)
        supertype:  relation_to_collection (*,collection)
           supertype:  relation_from/to_thing_of_common_kind (*)  this type permits to categorize relations according to their signatures and hence offers (i) a concise way to set essential exclusion relations, and (ii) a systematic and easy-to-follow categorization
              >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
           supertype:  what_relation (*)
              supertype:  wh-/how_relation (*)  this type permits to categorize relations according to the usual who/what/why/where/when/how questions ; this is a traditional but very subjective and ineffective way of categorizing relations 
                 >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
  supertype:  Identifier (? -> string)  to specify a reference that is unambiguous within a given context; SHOULD NOT BE USED IN WEBKB
     supertype:  name (?,?)
        supertype:  attributive_relation__attributiverelation (*)  like pm#binary_relation, this type mostly exists to categorize what cannot be categorized elsewhere
           supertype:  relation_playing_a_special_role (*)  this type permits to categorize relations according to their roles ; this is a traditional but quite subjective way of categorizing relations
              >part of:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type