Empty list
extend (2): 2 (idx: 0)
l.start
put_left (1): 1, 2 (idx: 2)
l.forth
put_left (6): 1, 2, 6 (idx: 4)
l.back
put_left (3): 1, 2, 3, 6 (idx: 4)
put_left (5): 1, 2, 3, 5, 6 (idx: 5)
l.back
put_left (4): 1, 2, 3, 4, 5, 6 (idx: 5)
Execution completed