I haven't given this enough thought to make up my mind, but having this guy hanging out with no methods and no inheritance feels a bit strange. It's related to `LogicalOperator` in some way, which isn't captured with the current implementation.

Mebbe this?

```php

namespace Ruler;

// all operators are, themselves, propositions:
interface Operator extends \Ruler\Proposition {}


namespace Ruler\Operator;

// there are (currently) three general categories of operators:
interface ArithmeticOperator extends \Ruler\Operator {}
interface ComparisonOperator extends \Ruler\Operator {}
interface LogicalOperator    extends \Ruler\Operator {}

// and, as an implementation detail, each of the concrete operators is either unary or binary:
abstract class BinaryOperator {}
abstract class UnaryOperator {}
```