Function dc#Identifier (? -> pm#string)  to specify a reference that is unambiguous within a given context; SHOULD NOT BE USED IN WEBKB
  subtype:  dl#identifier  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
  supertype:  pm#name (?,?)
     supertype:  pm#attributive_relation__attributiverelation (*)  like pm#binary_relation, this type mostly exists to categorize what cannot be categorized elsewhere
        supertype:  pm#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
           supertype:  pm#relation__related_thing__relatedthing___related_with (*)  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type

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