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Can a type have more than one immediate supertype? If not, why not?
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If it can, then can one define an abstract supertype of e.g. Float64? [ I sense from Make an existing type a child of an abstract type - #2 by Raf and How to make an abstract type as a super type of composite types that the answer is (still?) no, and I suspect that someone will advise me that this would be a very bad idea and (hopefully also) why]