Mathematica: POLICY: Webel: ADT pseudo classes: Multiple inheritance is supported via ONE (only) ADT-Class and one or more ADT-MemberInterfaces (which have no ADT-Method UpValues)

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The definition of a Webel ADT-MemberInterface is given under:

The image above shows a subset of a hierarchy including ADT-Classes XL$XL, XL$XL$Map, and XL$ResourcesMap, where XL$XL is a Webel ADT MemberInterface that defines membership in a family of classes that happen to be for handling OpenXML bundled XML spreadsheet files.

XL$Map has ADT signature XL$Map[$$_Association] and extends ADT$Map (with ADT template signature ADT$Map[$$_Association]), and also the MemberInterface XL$XL, which does nothing other than define membership.

Formally, the signature of XL$XL is XL$XL[$$:None], but that is typically not directly used (except for development testing):

The ADT-signature XL$XL[$$:None] is a blocking signature; the use of the single allowed match $$:None prevents it from being used for anything else:

i: get[XL$XL[None]]
o: None
i: get[XL$XL["will not work"]]

o: get[XL$XL["will not work"]]

(BTW error reporting for the above demonstration has been disabled.)

XL$XL$Map (which handles more restricted mapping to only XL$* classes) also extends ADT$Map and XL$XL.

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