## Variables and relations

The elementary relation in a decision diagram is equality between a variable and a value in its domain, for example `i == 5`

.

Normally, there is more than one variable, and relations use combined equalities through conjunction (the `and`

operator) and disjunction (the `or`

operator).

Example:

```
X ::= (i == 5 and j == 1)
or (i == 2 and j == 1)
```

The example shows a relation named `X`

that holds when `j`

is equal to `1`

, and `i`

must be either `2`

or `5`

.

The library itself provides constant relations `ONE`

to express `true`

(the relation that always holds), and `ZERO`

to express `false`

(the relation that never holds).