s1@(Sequence traits) longestCommonSubsequenceWith: s2@(Sequence traits)
[| lengths |
  lengths: (ArrayMD newWithDimensions: {s1 length `cache. s2 length `cache} defaultElement: 0).
  s1 doWithIndex: [| :elem1 :index1 |
    s2 doWithIndex: [| :elem2 :index2 |
      elem1 = elem2
        ifTrue: [lengths at: {index1 + 1. index2 + 1} put: (lengths at: {index1. index2}) + 1]
        ifFalse: [lengths at: {index1 + 1. index2 + 1} put:
          ((lengths at: {index1 + 1. index2}) max: (lengths at: {index1. index2 + 1}))]]].
  ([| :result index1 index2 |
   index1: s1 length.
   index2: s2 length.
   [index1 isPositive /\ index2 isPositive]
     whileTrue:
       [(lengths at: {index1. index2}) = (lengths at: {index1 - 1. index2})
          ifTrue: [index1: index1 - 1]
          ifFalse: [(lengths at: {index1. index2}) = (lengths at: {index1. index2 - 1})]
            ifTrue: [index2: index2 - 1]
            ifFalse: ["assert: (s1 at: index1 - 1) = (s2 at: index2 - 1)."
                      result nextPut: (s1 at: index1 - 1).
                      index1: index1 - 1.
                      index2: index2 - 1]]
   ] writingAs: s1) reverse
].