You can define a function to be recursive with the `rec`

keyword, so it can call itself.

```
# let rec fact n = match n with
| 0 -> 1
| n -> n * fact (n - 1);;
val fact : int -> int = <fun>
# fact 0;;
- : int = 1
# fact 4;;
- : int = 24
```

You can also define mutually recursive functions with the `and`

keyword, so they can call each other.

```
# let rec first x = match x with
| 1 -> 1
| x -> second (x mod 10)
and second x = first (x + 1);;
val first : int -> int = <fun>
val second : int -> int = <fun>
# first 20;;
- : int = 1
# first 12345;;
- : int = 1
```

Notice that the second function does not have the `req`

keyword.