Relation pm#disjoint (set_or_class,set_or_class)  like sumo#disjoint but from a a pm#set_or_class to another
  supertype:  relation_to_another_set_or_class  this category is needed to group SUMO relations between classes which cannot be subtype of pm#relation_from_type because their signatures curiously also involve sets
  subtype:  disjoint (set_or_class,set_or_class)  classes are exclusive/disjoint only if they share no instance (and hence no subtype), i.e. just in case the result of applying sumo#intersection_fn to them is empty
  subtype:  exclusive_class__exclusiveclas (class,class)  the 2 classes have no common subtype/instance; in WebKB, use the link '!'
     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 '/'