# -*- Mode: Python -*-

def circularize (l):
    pass

def pair_list (l):
    r = None
    special = None
    for i in range (len (l)):
        x = l[i]
        r = [x, r]
        if i == 0:
            special = r
    special[1] = r
    return r

def pair_list (l):
    r = [l[0], None]
    r[1] = r
    first = r
    for x in l[1:]:
        r = [x, r]
    first[1] = r
    return r

def pair_list (l):
    if len(l) == 1:
        return [l[0], l], l

def pair_list (l):
    if len(l) == 1:
        return [l[0], None]
    else:
        return [l[0], pair_list (l[1:])]

def _pair_list (l):
    if len(l) == 1:
        r = [l[0], None]
        return r, r
    else:
        result, tail = _pair_list (l[1:])
        result = [l[0], result]
        tail[1] = result
        return result, tail

def pair_list (l):
    result, tail = _pair_list (l)
    return result