Relation pm#relation_to_another_class (class,class+)
  supertype:  relation_from_class_to_collection  relation_to_another_set_or_class
  subtype:  sub_class_of__subclassof__super_class__superclas (class,class)  in WebKB, use the link '<'
  subtype:  equivalent_class (class,class)  in WebKB, use the link '='
  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 '/'
  subtype:  restricted_by (class,restriction)
  subtype:  disjoint_decomposition (class,class+)  a disjoint_decomposition of a class C is a set of mutually disjoint subclasses of C
     subtype:  partition (class,class+)  a partition of a class C is a set of mutually disjoint classes (a subclass partition) covering C; each instance of C is instance of exactly one of the subclasses in the partition
  subtype:  exhaustive_decomposition (class,class+)  an exhaustive_decomposition of a class C is a set of subclasses of C such that every instance of C is an instance of one of the subclasses in the set; note:  this does not necessarily mean that the elements of the set are disjoint (see sumo#partition - a partition is a disjoint exhaustive decomposition)
     subtype:  partition (class,class+)  a partition of a class C is a set of mutually disjoint classes (a subclass partition) covering C; each instance of C is instance of exactly one of the subclasses in the partition