def generate(self, *arg, **kw):
        """Call all plugins, yielding each item in each non-None result.
        """
        for p, meth in self.plugins:
            result = None
            try:
                result = meth(*arg, **kw)
                if result is not None:
                    for r in result:
                        yield r
            except (KeyboardInterrupt, SystemExit):
                raise
            except:
                exc = sys.exc_info()
                yield Failure(*exc)
                continue
Call all plugins, yielding each item in each non-None result.