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Code-Doku Joke

What do you call a song sung in an automobile?

Place each of the letters of CAST IRON exactly once in each row, column and 3×3 box. In addition to the letters, each row, column and 3×3 box will also contain one empty square. When you have completed the puzzle, the shaded squares will form the punch line of the joke read left to right, then top to bottom.

Difficulty: Fiendish

Here is a print-friendly version of this puzzle.

 

Please send responses to my email address is mathrec at this domain

Thanks,
Steve