Looks like you solved it, closing ~~

Hopefully fixed that, but unless otherwise specified, it is always better never to change the starting array.

So, there are 13 'O', 22 'A', 23 'W', 24 'H' in the input data, but 15 'O', 26 'A', 29 'W', 25 'H' in expected result. I don't get it... Maybe I misunderstood something? This is JavaScript version.

Input: [ [ 'A', 'H', 'O', 'H', undefined, 'A', 'H', 'A', 'W', undefined ],
[ 'A', 'W', 'H', 'A', 'O', 'A', 'W', 'H', 'W', 'O' ],
[ 'W', 'W', undefined, 'W', 'H', undefined, 'A', 'H', 'W', 'H' ],
[ 'W', undefined, 'O', 'W', undefined, 'A', 'W', 'A', 'A', 'H' ],
[ 'W', 'H', 'O', undefined, 'H', undefined, 'H', 'H', 'O', 'A' ],
[ 'O', undefined, 'H', 'H', 'W', 'A', 'H', undefined, 'A', undefined ],
[ 'O', 'H', 'W', 'A', 'O', 'A', 'H', 'H', 'W', 'A' ],
[ 'A', 'O', undefined, 'H', undefined, 'O', 'H', 'W', 'A', 'H' ],
[ 'H', 'W', 'A', undefined, 'W', 'A', 'W', 'W', 'O', 'O' ],
[ 'A','W',undefined,undefined,'W','H','W',undefined,undefined,'A' ] ]

Expected: [["O","O","O","O","O","O","O","O","O","O"],
["O","O","O","O","O","A","A","A","A","A"],
["A","A","A","A","A","A","A","A","A","A"],
["A","A","A","A","A","A","A","A","A","A"],
["A","W","W","W","W","W","W","W","W","W"],
["W","W","W","W","W","W","W","W","W","W"],
["W","W","W","W","W","W","W","W","W","W"],
["H","H","H","H","H","H","H","H","H","H"],
["H","H","H","H","H","H","H","H","H","H"],
["H","H","H","H","H"]],

instead got: [["O","O","O","O","O","O","O","O","O","O"],
["O","O","O","A","A","A","A","A","A","A"],
["A","A","A","A","A","A","A","A","A","A"],
["A","A","A","A","A","W","W","W","W","W"],
["W","W","W","W","W","W","W","W","W","W"],
["W","W","W","W","W","W","W","W","H","H"],
["H","H","H","H","H","H","H","H","H","H"],
["H","H","H","H","H","H","H","H","H","H"],
["H","H"],
[]]

Done! :) TY

Just for your description change the last sentence to
"Psst! Ask Pitagoras for more information, but don't tell him I sent you :)"