A rose tree is an ordered rooted tree in which each vertex can have any finite number of children in a rooted tree. A labelled Haskell representation is
data RTree a = Node a [RTree a]. Its tree fold has type (a -> [b] -> b) -> RTree a -> b: recursively fold the children, then combine their results with the current label. Unlike a binary tree, a rose tree has no fixed pair of child positions. New to topics? Read the docs here!