def triangles():
    L1 = [1]
    yield(L1)
    L2 = [1, 1]
    yield(L2)
    while 1:
        i = 0
        L3 = [1]
        while i < len(L2)-1:
            L3.append(L2[i] + L2[i+1])
            i = i + 1
        L3.append(1)
        yield(L3)
        L1, L2 = L2, L3