--- Stack (a) ---
000: []
--- Stack (b) ---
001: [[]]
000: 10
--- Stack (c) ---
000: [10,[[]],"a b c d",[]]
--- Stack (d) ---
001: 1
000: [10,[[]],"a b c d",[]]
--- Stack (e) ---
001: [10,"a b c d",[],[[]]]
000: 1
--- Stack (f) ---
000: 5