Function composition

Write a function comp that takes as input two functions f and g and returns a new function h that is the composition of f and g, that is:

h(x) = f (g x)

The function must have the following type:

comp : ('b -> 'c) -> ('a -> 'b) -> 'a -> 'c

where f : 'b -> c', g : 'a -> 'b, and the type of the result is 'a -> 'c.

Here are some unit tests:

let double x = x * 2;;
let square x = x * x;;

assert((comp square double) 3 = 36);;
assert((comp double square) 3 = 18);;

assert(comp (fun b -> if b then 0 else 1) (fun x -> x>0) 3 = 0);;
assert(comp (fun b -> if b then 0 else 1) (fun x -> x>0) 0 = 1);;