public void Visit(StyleSheet styleSheet)
 {
     foreach (var page in styleSheet.Pages)
     {
         page.Accept(this);
     }
     _qlQuestions.SymmetricExceptWith(_qlsQuestions);
     foreach (var question in _qlQuestions)
     {
         if (_qlsQuestions.Contains(question))
         {
             _messages.AddError("Question '" + question + "' is not in the QL program.");
         }
         else
         {
             _messages.AddError("Question '" + question + "' is in the QL program but not placed by the QLS program.");
         }
     }
 }
 public void Visit(Slider style)
 {
     if (!IsNumeric(_current))
     {
         _messages.AddError("Line: " + style.LineNumber + " | Slider widget cannot be applied to question of type " + _current + ".");
     }
 }