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.

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.
