Function pm#language (description_content/medium/container -> description_medium)
  supertype:  Language (description_content/medium/container -> entity)  to specify a language for the content of a resource
     supertype:  relation_from_description_content/medium/container (description_content/medium/container,*)
        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:  description_instrument__descriptioninstrument__descr_instrument__descrinstrument (description_content/medium/container,description_medium)
     supertype:  relation_from_description_content/medium/container (description_content/medium/container,*)