tap word in tube to show definition
disable board interaction when words are forming
allow swap executor to dealloc
since levels manager is a singleton, manage its observer differently to prevent a memory leak
merge boardController code into tileController, remove boardController
fix blank positions
extrapolate tile selection to prevent missing tiles in selection
extrapolate selection: use less code to do the same thing
do not allow extrapolated selection of non-touchable tile
add a comment
add a dlog to help debug an issue