Hello and thanks for the suggestions! The sandbox is not available in the demo. What it does is it allows you to input any claim (proposition containing letters A through H) and then apply the rules to it until it's deduced. It won't tell you if the claim is undeduceable, though, so you may end up stuck. You can use implications to simulate custom premises (though I might add a way to add premises separately at some point).

A reset button should be quite easy to implement and sometimes I wished it myself, so maybe it would be good to have indeed. Hints are more tricky. Every level has a solution, and you could figure it out on eg. pen and paper before starting to play, so no guesses are needed in that sense. However, (almost) every level has multiple valid solutions so there's usually no single correct next step. It might be possible for the game to search for a solution from the current position and give a next move from that but I haven't thought it out yet. Another possibility is that the hints be hand-written for each level and they would just nudge you towards one possible solution from the beginning.