you enumerate a collection, the integer part of each pair is a counter
for the enumeration, but is not necessarily the index of the paired value.
These counters can be used as indices only in instances of zero-based,
integer-indexed collections, such as <code>Array</code> and <code>ContiguousArray</code>. For
other collections the counters may be out of range or of the wrong type
to use as an index. To iterate over the elements of a collection with its
indices, use the <code>zip(_:_:)</code> function.</p>
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