code: public Formula cnf(final Collection<? extends Formula> clauses) {
      final LinkedHashSet<? extends Formula> ops = new LinkedHashSet<>(clauses);
      return this.constructCNF(ops);
    }
docs: Creates a new CNF from a collection of clauses.
  <p>
  ATTENTION: it is assumed that the operands are really clauses - this is not checked for performance reasons.
  Also no reduction of operands is performed - this method should only be used if you are sure that the CNF is free
  of redundant clauses.
  @param clauses the collection of clauses
  @return a new CNF