s@(Sequence traits) flatten
[
  [| :out | s flattenOn: out] writingAs: s
].

s@(Sequence traits) flattenOn: w@(WriteStream traits)
[
  s do: [| :value |
    (value is: s)
      ifTrue: [value flattenOn: w]
      ifFalse: [w nextPut: value]].
].