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boost::proto::fold_tree — A PrimitiveTransform that recursively applies the
          proto::fold<> transform to sub-trees
          that all share a common tag type.
// In header: <boost/proto/transform/fold_tree.hpp> template<typename Sequence, typename State0, typename Fun> struct fold_tree : proto::transform< fold_tree<Sequence, State0, Fun> > { // member classes/structs/unions template<typename Expr, typename State, typename Data> struct impl : proto::fold<Sequence, State0, recurse_if_<typename Expr::proto_tag, Fun> > ::template impl<Expr, State, Data> { }; };
            proto::fold_tree<> is useful for flattening trees into lists;
            for example, you might use proto::fold_tree<> to flatten an
            expression tree like a | b | c into a Fusion list like
            cons(c, cons(b, cons(a))).
          
            proto::fold_tree<> is easily understood in terms of a
            recurse_if_<> helper, defined as follows:
            
template<typename Tag, typename Fun> struct recurse_if_ : proto::if_< // If the current node has type type "Tag" ... boost::is_same<proto::tag_of<proto::_>, Tag>(), // ... recurse, otherwise ... proto::fold<proto::_, proto::_state, recurse_if_<Tag, Fun> >, // ... apply the Fun transform. Fun > {};
            With recurse_if_<> as defined above,
            proto::fold_tree<Sequence, State0, Fun>()(expr, state, data)
            is equivalent to:
            
proto::fold< Sequence, State0, recurse_if_<typename Expr::proto_tag, Fun> >()(expr, state, data).
It has the effect of folding a tree front-to-back, recursing into child nodes that share a tag type with the parent node.