Mathematica: Webel ADT pseudo classes: DEFINITION/CONVENTION: Functions that populate ADTs with reusable ADT-method UpValues are called 'definers' and include '$def' in the name after a Package scope indicator: Example: adt$def$ADT, my$def$MY$CleverList

A Webel definer is a top-level (non "ADT-method") function that accepts a unique signature pattern (which uniquely defines each Webel ADT pseudo class), and typically populates it with reusable UpValues.

Using the universal base ADT definer as an example):

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

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