Lookup (Right)

Implement the function

val lookup : ('a * 'b) list -> 'a -> 'b option

so that lookup l a is

• Some b if there is a pair of the form (a, b) in l. If there are multiple such pairs, the output should be the value associated with the rightmost such pair in l;
• None if there is no such pair.

let _ = assert (lookup [] 2 = None)
let _ = assert (lookup [(2, "2")] 2 = Some "2")
let _ = assert (lookup [(2, "2"); (3, "3"); (2, "4"); (3, "5")] 2 = Some "4")
let _ = assert (lookup [(2, "2"); (3, "3"); (2, "4"); (3, "5")] 4 = None)