inline.py

# The idea is from http://i.imgur.com/iVLQKSW.jpg

def sequence(n):
    value, past = 0, set()

    for c in range(n):
        t = value - c
        value = t if (t > 0 and t not in past) else (value + c)

        past.add(value)

        yield (value, c, past)

test_sequence.py

# The idea is from http://i.imgur.com/iVLQKSW.jpg

def sequence(n):
    value, past = 0, []

    for c in range(n):
        t = value - c
        if t > 0  and t not in past:
          value = t
        else:
          value += c

        past.append(value)

        yield (value, c, past)