code: private LinkedHashSet<Formula> gatherAppliedOperands(final NAryOperator operator) {
      final LinkedHashSet<Formula> applied = new LinkedHashSet<>();
      for (final Formula operand : operator) {
        applied.add(apply(operand, false));
      }
      return applied;
    }
docs: Gather the operands of an n-ary operator and returns its applied operands.
  @param operator the n-ary operator
  @return the applied operands of the given operator