prefixes2        ::  Int -> Forest -> [Forest]
prefixes2 x ts   =   [ Leaf x : rollup (take k ts) : drop k ts 
                     | k <- [1 .. length ts]]
prefixes2        ::  Int -> Forest -> [Forest]
prefixes2 x ts   =   [ Leaf x : rollup (take k ts) : drop k ts 
                     | k <- [1 .. length ts]]
prefixes2 x ts   =   [ Leaf x : rollup (take k ts) : drop k ts 
                     | k <- [1 .. length ts]]