cpp/algorithm/ranges/partial sort

@1@ Rearranges elements such that the range contains the sorted  smallest elements in the range.

@@ The order of equal elements is not guaranteed to be preserved. The order of the remaining elements in the range is unspecified.

@@ The elements are compared using the given binary comparison function and projected using  function object.

@2@ Same as, but uses as the range, as if using  as  and  as.

Return value
An iterator equal to.

Complexity
comparisons and twice as many projections, where is,  is.