def _first_stoppoint(self, irsb, extra_stop_points=None):
        """
        Enumerate the imarks in the block. If any of them (after the first one) are at a stop point, returns the address
        of the stop point. None is returned otherwise.
        """
        if self._stop_points is None and extra_stop_points is None and self.project is None:
            return None

        first_imark = True
        for stmt in irsb.statements:
            if type(stmt) is pyvex.stmt.IMark:  # pylint: disable=unidiomatic-typecheck
                addr = stmt.addr + stmt.delta
                if not first_imark:
                    if self.is_stop_point(addr, extra_stop_points):
                        # could this part be moved by pyvex?
                        return addr
                    if stmt.delta != 0 and self.is_stop_point(stmt.addr, extra_stop_points):
                        return addr

                first_imark = False
        return None
Enumerate the imarks in the block. If any of them (after the first one) are at a stop point, returns the address
        of the stop point. None is returned otherwise.