||Component type: function
template <class InputIterator, class Size, class OutputIterator>
OutputIterator copy_n(InputIterator first, Size count,
Copy_n copies elements from the range [first, first + n) to the
range [result, result + n). That is, it performs the assignments
*result = *first, *(result + 1) = *(first + 1), and so on.
Generally, for every integer i from 0 up to (but not including)
n, copy_n performs the assignment *(result + i) = *(first + i).
Assignments are performed in forward order, i.e. in order of
increasing n. 
The return value is result + n.
Declared in algo.h. The implementation is in
Requirements on types
InputIterator is a model of Input Iterator.
OutputIterator is a model of Output Iterator.
Size is an integral type.
InputIterator's value type is convertible to a type in
OutputIterator's set of value types.
n >= 0.
[first, first + n) is a valid range.
result is not an iterator within the range [first, first + n).
[result, result + n) is a valid range.
Linear. Exactly n assignments are performed.
iota(V.begin(), V.end(), 1);
copy_n(V.begin(), V.size(), L.begin());
assert(equal(V.begin(), V.end(), L.begin()));
Copy_n is almost, but not quite, redundant. If first is an
input iterator, as opposed to a forward iterator, then the
copy_n operation can't be expressed in terms of copy.
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