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

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

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

`proto::callable`
This(Seq, N)
fusion::result_of::at< typename boost::remove_reference<Seq>::type , typename boost::remove_const<typename boost::remove_reference<N>::type>::type >
typename fusion::result_of::at< Seq, N >::type `Seq &` `N const &`

`fusion::at<N>(seq)`

typename fusion::result_of::at< Seq const, N >::type `Seq const &` `N const &`

`fusion::at<N>(seq)`