'enable tracing of hook calls and return an undo function.'

def enable_tracing(self):

    hooktrace = self.hook._trace
    def before(hook_name, methods, kwargs):
        hooktrace.root.indent += 1
        hooktrace(hook_name, kwargs)
    def after(outcome, hook_name, methods, kwargs):
        if (outcome.excinfo is None):
            hooktrace('finish', hook_name, '-->', outcome.result)
        hooktrace.root.indent -= 1
    return self.add_hookcall_monitoring(before, after)
