boost::proto::flatten — A function that returns a "flattened" view of a Proto expression tree.
template<typename Expr> typename proto::result_of::flatten< Expr const >::type flatten(Expr const & expr);
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).