Mathematica: POLICY: Webel: ADT pseudo classes that define membership of ADT-classes in a domain package have ADT-signature MemberInterface[$$:None] and MUST NOT populate ADT-method UpValues in their definers.

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Such ADTs are called ADT MemberInterfaces in the Webel recipe. They may participate in multiple inheritance ONLY because they DO NOT define (generate) any ADT-method UpValues. For an example visit:
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).
IMPORTANT: Webel: The definer of an ADT MemberInterface is NEVER called by a client sub-class definer if that client sub-class is involved in multiple inheritance via another ADT-Class (that generates ADT-Method UpValues)!

It may be called by a client sub-class definer that only wishes to inherit the functionality of the universal base ADT whilst "inheriting" an otherwise passive membership role, and otherwise defines all of its own ADT-method UpValues.

Note that an ADT-Class participating in multiple inheritance will get those base ADT ADT-method UpValues (against its own injected signature) through the one (1) allowed concrete ADT it extends.

You could if you really wish include ADT-method UpValues in a an ADT MemberInterface if you really enjoy really, really hard to trace bugs. A better and more enjoyable approach (even if it is sometimes a bit tedious) is:

Mathematica: Webel: ADT pseudo classes: TIP: Prefer the Decorator Pattern over multiple inheritance of ADT-Method UpValues! (But not because "inheritance is evil", it isn't.)
Relates to
[STUB] TRAIL: Mathematica: The Webel recipe for Abstract Data Type (ADT) pseudo classes with inheritance for the Wolfram Language
Related notes
[CONVENTION, NAMING]{STRICT} Mathematica: NAMING CONVENTION: Webel: ADT pseudo classes: An UpValue that can be invoked on an ADT accepting its unique 'signature' pattern as (at least) 1st argument is called an "ADT-method"'. They are NOT methods in the general object-oriented sense!
[CONVENTION, POLICY]{STRICT} Mathematica: Webel: ADT pseudo classes: POLICY: Every named concrete Abstract Data Type (ADT) class has a ONE unique "signature" (which is the pattern passed to the "definer" functions). To vary a signature, define another unique ADT class name.
[CONVENTION, POLICY]{STRICT} Mathematica: Webel: ADT pseudo classes: POLICY: The Abstract Data Types (ADTs) are stateless functional (although inheritance and overrides are supported), with no caching by default (although there is nice optional caching using CreateUUID[] and Once[]).
[POLICY]{STRICT} Mathematica: Webel: ADT pseudo classes: POLICY: The expression '$$' is reserved as the primary named parameter for the defining pattern of all ADT classes that extend the universal base 'ADT' class.
[POLICY]{STRICT} Mathematica: Webel: ADT pseudo classes: There is an 'ADT' universal base class that has no supers. It "blesses" every other sub-class ADT by populating it with some common ADT-method UpValues against its signature pattern via the 'adt$def$All[]' definer.
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