| AssociativePlFormula | This class captures the common functionalities of formulas with an associative
 operation like conjunction, disjunction, etc. | 
| Conjunction | This class represents a conjunction in propositional logic. | 
| Contradiction | A contradictory formula. | 
| Disjunction | This class represents a disjunction in propositional logic. | 
| Equivalence | This class models equivalence of propositional logic. | 
| ExclusiveDisjunction | This class represents an exclusive disjunction (XOR) in propositional logic. | 
| Implication | This class models the implication of propositional logic. | 
| Negation | This class models classical negation of propositional logic. | 
| PlBeliefSet | This class represents a knowledge base of propositional formulae. | 
| PlFormula | This class represents the common ancestor for propositional formulae. | 
| PlPredicate | A specialized predicate for propositional logic that only allows an identifier
 but has no arguments and therefore has an arity of zero. | 
| PlSignature | This class captures the signature of a specific propositional language. | 
| Proposition | This class represents a simple proposition in propositional logic. | 
| SpecialFormula | This class captures the common functionalities of the special
 formulas tautology and contradiction. | 
| Tautology | A tautological formula. |