public string Visit(DisjunctionFormula disjunctionFormula)
 {
     return($"({disjunctionFormula.LeftSubformula.Apply(this)} ∨ {disjunctionFormula.RightSubformula.Apply(this)})");
 }
 internal DisjunctionTableau(DisjunctionFormula formula, TruthLabel truthLabel)
 {
     Formula    = Disjunction = formula;
     TruthLabel = truthLabel;
 }