Copy current_context into `lines` down up until lineno

def _filldown(self, lineno):
        """
        Copy current_context into `lines` down up until lineno
        """
        if self.line > lineno:
            # XXX decorated functions make us jump backwards.
            # understand this more
            return

        self.lines.extend(
            self.current_context for _ in range(self.line, lineno))
        self.line = lineno