'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.'

def _first_stoppoint(self, irsb):

    if (self._stop_points is None):
        return None
    first_imark = True
    for stmt in irsb.statements:
        if isinstance(stmt, pyvex.stmt.IMark):
            addr = (stmt.addr + stmt.delta)
            if ((not first_imark) and (addr in self._stop_points)):
                return addr
            first_imark = False
    return None
