Matteo, sorry for the belated reply. Of course, you must jump out of the loop as soon as you find the king, so you should write: (while (and (on-board? next) (not-flag? found))...
The parse error is that you write (verify found?). It should be (verify (flag? found)).
/Mats
(while (and (on-board? next) (not-flag? found))...
The parse error is that you write (verify found?). It should be (verify (flag? found)).
/Mats