code: @Override
      protected Operand createAndExpression(final Operand leftExpression, final Operand rightExpression) {
          if (leftExpression == null || rightExpression == null) {
              return null;
          }
  
          final Set<Operand> operands = new HashSet<Operand>();
          operands.add(leftExpression);
          operands.add(rightExpression);
  
          return new Operand(Operator.AND, operands);
      }
docs: {@inheritDoc}