lits.clear();
lits.add(literals.get(i));
lits.addAll(literals.subList(0, i));
lits.addAll(literals.subList(i + 1, literals.size()));
Chain cont = new Chain(lits);
cont.setProofStep(new ProofStepChainContrapositive(cont, this));
contrapositives.add(cont);
}
return contrapositives;
}