Change management for local theorem links

[till]

For each local axiom in the source of the link, compute the set of used symbols, and intersect this with the set of symbols mapped by the morphism. Then check if the new morphism behaves differently from the old one on this intersection.

[maeder]

Maybe it is easier to check if the translated sentence changes, if the corresponding morphism has changed, since an aiom has to be re-translated anyway if it changes.