Make any Proto expression a valid Fusion sequence A PolymorphicFunctionObject type that returns a "flattened" view of a Proto expression tree. A PolymorphicFunctionObject type that returns a "flattened" view of a Proto expression tree. For a tree with a top-most node tag of type T, the elements of the flattened sequence are determined by recursing into each child node with the same tag type and returning those nodes of different type. So for instance, the Proto expression tree corresponding to the expression a | b | c has a flattened view with elements [a, b, c], even though the tree is grouped as ((a | b) | c). The resulting view is a Fusion Forward Sequence. proto::callable This(Expr) result< This(Expr const &) > This(Expr &) proto::result_of::flatten< Expr > typename proto::result_of::flatten< Expr >::type const Expr & Returns a Fusion Forward Sequence representing a flattened view of expr. typename proto::result_of::flatten< Expr const >::type const Expr const & Returns a Fusion Forward Sequence representing a flattened view of expr. Metafunction that computes the return type of proto::flatten() unspecified A Fusion Forward Sequence typename proto::result_of::flatten< Expr >::type const Expr & typename proto::result_of::flatten< Expr const >::type const Expr const & A function that returns a "flattened" view of a Proto expression tree. For a tree with a top-most node tag of type T, the elements of the flattened sequence are determined by recursing into each child node with the same tag type and returning those nodes of different type. So for instance, the Proto expression tree corresponding to the expression a | b | c has a flattened view with elements [a, b, c], even though the tree is grouped as ((a | b) | c). The returned view is a Fusion Forward Sequence.