(VAR u v x y z )
(RULES 
        merge(nil, y) -> y
        merge(x, nil) -> x
        merge(.(x, y), .(u, v)) -> if(<(x, u), .(x, merge(y, .(u, v))), .(u, merge(.(x, y), v)))
        ++(nil, y) -> y
        ++(.(x, y), z) -> .(x, ++(y, z))
        if(true, x, y) -> x
        if(false, x, y) -> x
        
)