advanceEditState (EditState (Just i, pos) vars) SEBackSpace =
    let newVars = M.adjust (backspace pos) i vars
    in EditState (Just i, pos - 1) newVars
advanceEditState (EditState (Just i, pos) vars) SEBackSpace =
    let newVars = M.adjust (backspace pos) i vars
    in EditState (Just i, pos - 1) newVars
advanceEditState (EditState (Just i, pos) vars) SEBackSpace =
    let newVars = M.adjust (backspace pos) i vars
    in EditState (Just i, pos - 1) newVars