[boost] [range] transform surprise

I'm surprised that boost::range::transform args are template< class SinglePassRange1, class OutputIterator, class UnaryOperation

OutputIterator transform(const SinglePassRange1& rng, OutputIterator out, UnaryOperation fun); I would have expected: template< class SinglePassRange1, class SinglePassRange2, class UnaryOperation

OutputIterator transform(const SinglePassRange1& rng, SinglePassRange2& out, UnaryOperation fun); Why the asymmetry?