Compose two functions. `f .: g` is similar to `f . g`
except that `g` will be fed *two* arguments instead of one
before handing its result to `f`.
This function is defined as
*two* arguments (the function *and* the list)
will be given to `map` before the result is passed to
`concat`. This is equivalent to:

(f .: g) x y = f (g x y)Example usage:

concatMap :: (a -> [b]) -> [a] -> [b] concatMap = concat .: mapNotice how

concatMap f xs = concat (map f xs)

Equivalent to .:
The pattern of appending asterisks is straightforward to extend to
similar functions: (compose2 = .*, compose3 = .**, etc). However,
`.:` has been commonly adopted amongst Haskellers, and the need
for compose3 and beyond is rare in practice.

Compose composed with compose operator.

(f ... g) x y === f (g x y)

Multivariable composition.

f .: g ≡ (f .) . g ≡ \c d -> f (g c d)