Returns the current hook for `addr`.

        :param addr: An address.

        :returns:    None if the address is not hooked.

def hooked_by(self, addr):
        """
        Returns the current hook for `addr`.

        :param addr: An address.

        :returns:    None if the address is not hooked.
        """

        if not self.is_hooked(addr):
            l.warning("Address %s is not hooked", self._addr_to_str(addr))
            return None

        return self._sim_procedures[addr]