fork              ::  (Int, Tree) -> (Int, Tree) -> (Int, Tree)
fork (a,u) (b,v)  =   (1 + (a `max` b), Fork u v)
fork              ::  (Int, Tree) -> (Int, Tree) -> (Int, Tree)
fork (a,u) (b,v)  =   (1 + (a `max` b), Fork u v)
fork (a,u) (b,v)  =   (1 + (a `max` b), Fork u v)