function to be used for the new scope, or ``None`` if the scope shouldn't be
traced.
- The local trace function should return a reference to itself (or to another
- function for further tracing in that scope), or ``None`` to turn off tracing
- in that scope.
+ The local trace function should return a reference to itself, or to another
+ function which would then be used as the local trace function for the scope.
If there is any error occurred in the trace function, it will be unset, just
like ``settrace(None)`` is called.