def extend_overstrand_backward(overstrand, start_cep):
    """
    Starting at a crossing (under or over), extend overstrand in the backwards
    direction until you hit another undercrossing.
    """
    cep = start_cep.previous()
    overstrand.insert(0, start_cep)
    while start_cep.is_over_crossing():
        overstrand.insert(0, cep)
        cep = cep.previous()
        if cep == overstrand[-1]:
            break
