def _join_strips(strips):
    """concatenate strips"""
    strips = iter(strips)
    strip = next(strips, [])
    for s in strips:
        if len(strip) % 2 == 1:
            strip += [strip[-1], s[0], s[0]]
        else:
            strip += [strip[-1], s[0]]
        strip += s
    return strip
