prefixes x t@(Fork u v)  =   [Fork (Leaf x) t] 
                         ++  [Fork u' v | u' <- prefixes x u ]
prefixes x t@(Fork u v)  =   [Fork (Leaf x) t] 
                         ++  [Fork u' v | u' <- prefixes x u ]
prefixes x t@(Fork u v)  =   [Fork (Leaf x) t] 
                         ++  [Fork u' v | u' <- prefixes x u ]