Multiple times I've read dialogs like this example:
— This conjecture hasn't been proved.
— Yes it has been proved in 2003.
This seems odd to me: the answerer first says "yes", but proceeds to contradict his own beginning. I'd suppose the answer should rather have been something like "No, it has been proved in 2003.", meaning "You're wrong, it has been proved in 2003."
Is it actually normal usage of "yes" in English?