Definition of the ``proto::pass_through<>`` transform, which is the default transform of all of the expression generator metafunctions such as ``proto::unary_plus<>``, ``proto::plus<>`` and ``proto::nary_expr<>``.

proto::deduce_domain
`proto::transform`< pass_through<Grammar, Domain> >
A PrimitiveTransform that transforms the child expressions of an expression node according to the corresponding children of a Grammar. The resulting expression is in the specified domain.

Given a Grammar such as ``proto::plus`<T0, T1>`, an expression type that matches the grammar such as ``proto::plus`<E0, E1>::type`, a state `S` and a data `D`, the result of applying the `proto::pass_through<`proto::plus`<T0, T1> >` transform is:

    proto::plus<
      boost::result_of<T0(E0, S, D)>::type,
      boost::result_of<T1(E1, S, D)>::type
    >::type

The above demonstrates how child transforms and child expressions are applied pairwise, and how the results are reassembled into a new expression node with the same tag type as the original.

The `Domain` template parameter determines which domain the resulting expression should be in. If it is ``proto::deduce_domain``, which is the default, the resulting expression is in the same domain as the expression passed in. Otherwise, the resulting expression is in the specified domain. Practically, that means the specified domain's generator is used to post-process the resulting expression.

The explicit use of `proto::pass_through<>` is not usually needed, since the expression generator metafunctions such as ``proto::plus`<>` have `proto::pass_through<>` as their default transform. So, for instance, these are equivalent:

- ``proto::when`< `proto::plus`<X, Y>, proto::pass_through< `proto::plus`<X, Y> > >`
- ``proto::when`< `proto::plus`<X, Y>, `proto::plus`<X, Y> >`
- ``proto::when`< `proto::plus`<X, Y> > // because of proto::when<class X, class Y=X>`
- ``proto::plus`<X, Y> // because plus<> is both a grammar and a transform`

For example, consider the following transform that promotes all `float` terminals in an expression to `double`.

    // This transform finds all float terminals in an expression and promotes
    // them to doubles.
    struct Promote :
      proto::or_<
        proto::when<proto::terminal<float>, proto::terminal<double>::type(proto::_value) >,
        // terminal<>'s default transform is a no-op:
        proto::terminal<proto::_>,
        // nary_expr<> has a pass_through<> transform:
        proto::nary_expr<proto::_, proto::vararg<Promote> >
      >
    {};

`proto::transform_impl`<Expr, State, Data>
For each N in [0,Expr arity), for exposition only
typename proto::result_of::child_c<Grammar, N>::type
For each N in [0,Expr arity), for exposition only
typename proto::result_of::child_c<Expr, N>::type
For each N in [0,Expr arity), for exposition only
typename boost::result_of<GN(EN,State,Data)>::type
For exposition only
typename Expr::proto_tag
For exposition only
boost::is_same<Domain, `deduce_domain`>
For exposition only
typename Expr::proto_domain
For exposition only
typename mpl::if_<Deduce, DD, Domain>::type
For exposition only
typename D::proto_generator
For exposition only
`proto::listN`<R0,...RN>
For exposition only
`proto::expr`<T, A>
For exposition only
`proto::basic_expr`<T, A>
For exposition only
typename mpl::if_<`proto::wants_basic_expr`<G>, BE, E>::type
typename boost::result_of<D(expr_type)>::type
result_type `typename impl::expr_param` `typename impl::state_param` `typename impl::data_param`

``proto::matches`<Expr, Grammar>::value` is `true`.

    D()(expr_type::make(
      G0()(proto::child_c<0>(expr), state, data),
      ...
      GN()(proto::child_c<N>(expr), state, data)
    ))