Relation pm#file_or_file_element (?,description_container)
  supertype:  Description  to specify an account of the content of a resource; in WebKB, pm#topic or pmdescr SHOULD BE USED INSTEAD
  subtype:  file (?,document_element)
     subtype:  home_page (?,document_element)
  subtype:  file_element (?,document_element)
  subtype:  descr_in__descrin (?,description_container)  when a thing t has a description stored in a description container dc, there is a relation pm#descr_in from t to dc
     subtype:  descr_container__description_support___descr_support (description,description_container)  permits to associate a statement with an object on which it is represented