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A Set is a sequence of unique values. This implementation uses the same hash
table as Ds\Map, where values are used as keys and the
mapped value is ignored.
Strengths
Values can be any type, including objects.
Supports array syntax (square brackets).
Insertion order is preserved.
Automatically frees allocated memory when its size drops low enough.
add(),
remove() and
contains() are all O(1).
Weaknesses
Doesn't support
push(),
pop(),
insert(),
shift(), or
unshift().
get() is O(n) if there are deleted values
in the buffer before the accessed index, O(1) otherwise.