def extend_overstrand_forward(overstrand, end_cep):
    """
    Starting at a crossing (under or over), extend overstrand until you hit
    the next undercrossing.
    """
    cep = end_cep.next()
    overstrand.append(end_cep)
    while cep.is_over_crossing():
        overstrand.append(cep)
        cep = cep.next()
        if cep == overstrand[0]:
            break
