s1@(Sequence traits) longestCommonSubsequenceWith: s2@(Sequence traits)
[
  s1 isEmpty \/ s2 isEmpty ifTrue: [^ {}].
  s1 last = s2 last
    ifTrue: [(s1 allButLast longestCommonSubsequenceWith: s2 allButLast) copyWith: s1 last]
    ifFalse: [| x y |
              x: (s1 longestCommonSubsequenceWith: s2 allButLast).
              y: (s1 allButLast longestCommonSubsequenceWith: s2).
              x length > y length ifTrue: [x] ifFalse: [y]]
].