---
[[[["a", "b"], "c"], "d"]]
[[["a", "b"], "c"], "d"]
[["a", "b"], "c", "d"]
["a", "b", "c", "d"]
---
[[["a", "b"], ["c", "d"]]]
[["a", "b"], ["c", "d"]]
["a", "b", ["c", "d"]]
L=a, R=a
---
["b", "c", "d"]
---
["b", ["c", "d"]]
L=b, R=b
---
["c", "d"]
---
[["c", "d"]]
["c", "d"]
L=c, R=c
---
["d"]
---
["d"]
L=d, R=d
---
[]
---
[]
L=, R=