lass
of <code>NSArray</code> can become an <code>Array</code>, there are no guarantees about
representation or efficiency in this case (see also
<code>ContiguousArray</code>).  Since <code>NSArray</code> is immutable, it is just as
though the storage was shared by some copy: the first in any
sequence of mutating operations causes elements to be copied into
unique, contiguous storage which may cost <code>O(N)</code> time and space,
where <code>N</code> is the length of the array (or more, if the underlying
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