Other common functions are `fst`, which takes two arguments and returns the first of them; `snd`, which takes two arguments and returns the second of them; and `swap`, which takes two arguments and returns them both but with their positions swapped. A fourth function is `dup`, which takes one argument and returns it twice.
These functions can be defined like this:
+<a id=functions></a>
+
let
fst (x, y) = x;
snd (x, y) = y;