a -> a
a -> b -> a
a -> (a -> b) -> b
(a -> b) -> (a -> b -> c) -> (a -> c)
(a -> b) -> (b -> c) -> (a -> c)
(a | b) -> !a -> b
(a -> b) -> (!a -> b) -> b
(!a -> b) -> (!a -> !b) -> a
a | b -> (a -> c) -> (b -> c) -> c
a | b | c -> (a -> d) -> (b -> d) -> (c -> d) -> d