:: (c -> d) -> (a -> b -> c) -> a -> b -> d -is:exact -is:exact -package:fitspec -package:composition

Binary composition: pass two args to the right argument before composing.
(f .: g) x y = f (g x y)
or,
f .: g = curry (f . uncurry g)
This is the same as the common idiom (f .) . g but more easily extended to multiple uses, due to the fixity declaration.
Multivariable composition.
f .: g ≡ (f .) . g ≡ \c d -> f (g c d)
Wrap the result of a function applied to 2 arguments.