originalSentences.add(aSentence);
// Convert the sentence to CNF
CNF cnfOfOrig = cnfConverter.convertToCNF(aSentence);
for (Clause c : cnfOfOrig.getConjunctionOfClauses()) {
c.setProofStep(new ProofStepClauseClausifySentence(c, aSentence));
if (c.isEmpty()) {
// This should not happen, if so the user
// is trying to add an unsatisfiable sentence
// to the KB.
throw new IllegalArgumentException(