Contains vs is in

[hh]

(LC Builder 'contains' vs 'is in')
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TMHO this abstract writing of Ali's examples is easy to remember,
the different "level" of the comparison is visible by the brackets.

x "is in" A if and only if A "contains" [x].
(x "is in" A and y "is in" A) if and only if A contains [x,y].
etc.
where A is a list, x,y is any.


>> Definitions. Let A and B be lists.
>> x "is in" A if and only if x is an element of A.
>> B "contains" A if and only if every element of A "is in" B.