**CLP(FD) constraints** (*Finite Domains*) implement arithmetic over **integers**. They are available in all serious Prolog implementations.

There are two major use cases of CLP(FD) constraints:

- Declarative integer arithmetic
- Solving combinatorial problems such as planning, scheduling and allocation tasks.

Examples:

```
?- X #= 1+2.
X = 3.
?- 3 #= Y+2.
Y = 1.
```

Note that if `is/2`

were to be used in the second query, an instantiation error would occur:

```
?- 3 is Y+2.
ERROR: is/2: Arguments are not sufficiently instantiated
```