Fixed Config::hasStack : it was not working with index 0

reading_machine/src/Config.cpp

@@ -393,7 +393,7 @@ bool Config::hasHistory(int relativeIndex) const
 
 bool Config::hasStack(int relativeIndex) const
 {
-  return relativeIndex > 0 && relativeIndex < (int)stack.size();
+  return relativeIndex >= 0 && relativeIndex < (int)stack.size();
 }
 
 Config::String Config::getState() const