Mathematica: Webel ADT pseudo classes: A tricky POLICY & CONVENTION: Client packages MUST access the special "wrapped" primary named ADT-parameter '$$' via $ContextAliases! The recommended alias is "A`" (which stands of course for ADT).

This is the big trick for making the Webel recipe for ADT pseudo classes easily reusable and easy to code against, and works together with: The pattern is (adapt as needed):

BeginPackage["MY`",{
   "Webel`ADT`ADT`",
   (* ... other required ...*)
   }
]

$ContextAliases["A`"] =.; (*!!*)
$ContextAliases["A`"] = "Webel`ADT`ADT`;

Note how the $ContextAliases gets cleared just for A` first! Otherwise you'll get a message if you use the same trick in another package.

Example: A package-specific ADT sub-class MY$String within client package MY`:

BeginPackage["MY`ADT`",{
ClearAll[MY$String];
pattern = adt$def$ADT[
    MY$String,
    "Simple ADT String handler example",
    MY$String[A`$$_String],
    True (*!isAllSuper*)
];
MY$String /: stringLength[pattern] := StringLength[A`$$];

Note the use of A`$$ everywhere throughout the client package! Dr Darren says:

I'm really not into claims of "magical" serendipity, but having the term Abstract Data Type start with an 'A' is rather handy.

If you examine Context[$$] you'll find it is in fact Webel`ADT`ADT`$$ (or as adapted). This enables one to pass a 'pattern' involving '$$' cleanly to super definers.