Edit Script: + - =

Once you have the LCS, generate an edit script: a sequence of operations that transforms A into B.

A: ABCBDAB
B: BDCAB
LCS: BCAB

Edit script:
  - A    (delete from A)
    B    (keep)
  - C    (delete from A)
    B    (keep)
  + D    (insert from B)
  - D    (delete from A)
  - A    (delete from A)...

Actually let's redo with LCS=BDAB:
  - A
    B
  - C
    B
    D
  - A
  + (none — implicit)
    A
    B
  + (none)

Implementation: walk the DP backtrack, emitting:

• KEEP (=) when chars match
• DELETE (-) when stepping up (consumed A's char)
• INSERT (+) when stepping left (consumed B's char)