Mathematica: Webel ADT pseudo classes: There is an 'ADT' universal base that has no supers. It "blesses" every other sub-class ADT by populating it with some common ADT-Method UpValues against a signature pattern via the 'adt$def$All[]' definer function.

adt$def$ADT[
  class_Symbol,
  $usage_String,
  pattern_,
  isAllSuper:(True|False):False
] := Module[{ ...},
...
  class /: get[pattern] := $$;
  class /: head[pattern] := Head[$$];
...
  pattern
];

Its definer can be used on-the-fly to create new ADT sub-classes:

pattern = adt$def$ADT[MY$ADT$String,
  "An ADT that carries a String",
  MY$ADT$String[$$_String],
  True
];
MY$ADT$String /: stringLength[pattern] := StringLength[$$]; 

my$adt$s = MY$ADT$String["string1"];

stringLength[my$adt$s]

7