Function pm#closed_exclusion (type -> type)  the '/' link in the FT notation:  the two linked types either are respectively identical to pm#thing and pm#nothing (they are "complement types") or they subtype a same type and form a complete subtype partition
  supertype:  exclusive_type__exclusivetype  in WebKB, use the '!' link
  subtype:  complement_type (type -> type)  a supertype of owl#complement_of which can only connect RDFS/OWL classes
     subtype:  complement_class (class -> class)  if something is not in one of the classes, then it is in the other, and vice versa; in WebKB, use the link '/'