To explain further, the `receive` keyword is a bit like a switch statement (but actually a pattern matcher) for incoming messages.

Here they made a new server that takes an return process (From) and a number (N). The exclamation point sends the result back to the return process.

    factorial_server() ->
        receive
           {From, N} ->
               From ! factorial(N),
               factorial_server()
        end.

    factorial(0) -> 1;
    factorial(N) -> N * factorial(N-1).

This code then spawns the server, sends a message to that server to become a factorial server, then tells that server to send it back a message with the factorial of 50. It then specifies it's own message listener that takes whatever it receives and returns it.

    test() ->
        Pid = spawn(fun universal_server/0),
        Pid ! {become, fun factorial_server/0},
        Pid ! {self(), 50},
        receive
            X -> X
        end.

A couple of the major advantages of Erlang its distributed parallel nature, and also hot code update. Which happens in `Pid ! {become, fun factorial_server/0}` where it overides the receive loop of universal_server with that of the factorial_server. Though I think proper hot code update doesn't work like this