# Natural numbers

The natural numbers are the numbers 0, 1, 2, etc. The term is generally used to indicate a specific technique of representing natural numbers as either zero or the successor of some other natural number - 0, s(0), s(s(0)), etc - as in Peano arithmetic, a technique also sometimes referred to as unary numbers.

## Natural numbers in Twelf

Natural numbers in Twelf are usually defined in a similar way. Mathematically, natural numbers can be defined as zero or the successor of some other natural number: This representation translates easily into Twelf:

```nat: type.
z: nat.
s: nat -> nat.```

The first line declares that nat is a type. The second line declares z (zero) to be an object of type nat, and the third line declars s (successor) to be a type constructor that takes an object N of type nat and produces another object (s N) of type nat.

## Addition of natural numbers in Twelf

The addition of these natural numbers is defined by the judgment , where , , and are natural numbers. In the definition below, capital letters stand for metavariables that can range over all natural numbers.  These judgments also translate cleanly into Twelf:

```plus: nat -> nat -> nat -> type.
p-z: plus z N N.
p-s: plus (s N1) N2 (s N3)
<- plus N1 N2 N3.```

The first line defines the judgment, declaring plus to be a type family indexed by three terms of type nat.

The second line declares that for any natural number N, p-z is an object of type plus z N N, which corresponds to the axiom p-z above. The N is an implicit parameter - it is treated as a bound variable by Twelf, which you can see by looking at Twelf's output after checking the above code.

The third line says that p-s is a type constructor that, given an object D of type plus N1 N2 N3 (where N1, N2, and N3 are all implicit parameters that can be treated as metavariables), produces an object, p-s D, with type plus (s N1) N2 (s N3). This corresponds to the rule p-s, which given a derivation of allows us to conclude .

Consider this derivation which encodes the fact that : This can be represented in Twelf by applying the type constructor p-s to the object p-z twice:

`2+1=3 : plus (s (s z)) (s z) (s (s (s z))) = p-s (p-s p-z).`
```Twelf 1.7.1+ (r1896, built 05/05/15 at 12:56:43 on yazoo.plparty.org)
2+1=3 : plus (s (s z)) (s z) (s (s (s z))) = p-s (p-s p-z).

%% OK %%```