At each position, we can take one of the following editing operations: (i) KEEP to keep the word and go to the next word, (ii) DELETE to delete the word and go to the next word, (iii) IN-SERT(WORD) to insert a new WORD and stay in that position, or (iv) STOP to terminate the process.
the size of the operation set equals the size of the target vocabulary plus three, where we add the symbols KEEP, DELETE, and STOP as new tokens.