There are 2 issues here:

• Callable is highly dependent to getDeclaringClass(). Not sure how to deal with it: whether we should have a lot of methods in Callable to try to solve the issue or consider Callable as a JavaBean only thing.
• Let's say we move getDeclaringClass() from Constrainable to Callable: we still have a few issues to solve: we need an identifier for the declaring element: this is easily solvable. But we also have a big issue in ValidatorImpl: