//! Unary and binary operators for propositional formulas.
/// There are two types of operators in propositional formulas:
/// 1. `UnaryOperator`: arity 1.
/// 2. `BinaryOperator`: arity 2.
/// There is only one basic unary operator, the negation (logical NOT) operator.
/// There are four basic binary operators:
/// 1. Logical AND.
/// 2. Logical OR.
/// 3. Implication.
/// 4. Biimplication.