Includes Proto callable ``boost::proto::functional::begin``.

A PolymorphicFunctionObject type that invokes the `boost::begin()` accessor function on its arguments.

A PolymorphicFunctionObject type that invokes the `boost::begin()` accessor function on its arguments.

`proto::callable`
This(Range)
boost::range_iterator< typename boost::remove_reference<Range>::type >
typename boost::range_iterator< Range >::type `Range &`

`boost::begin(rng)`

typename boost::range_iterator< Range const >::type `Range const &`

`boost::begin(rng)`