Relation pm#equal (?,?)  "=" in KIF; true if the 1st argument is identical to the 2nd
  exclusion:  different
  supertype:  equivalence_relation__equivalencerelation  this category only serves structuration purposes: it is instance of pm#equivalence_relation_type which is not instance of pm#class_of_inheritable_relation_type
  instance of:  binary_predicate_type  equivalence_relation_type  type_of_relation_extended_to_quantities
  equal:  equal (pm)
  subtype:  same_as (?,?)  MORE PRECISE TYPES THAN THIS ONE SHOULD BE USED
     subtype:  same_type_as (type,type)
        subtype:  equivalent_class (class,class)  in WebKB, use the link '='
        subtype:  equivalent_property (binary_relation_type,binary_relation_type)  in WebKB, use the link '='
     subtype:  same_individual_as (?,?)