function draw_crate_to_move()
  if crate_to_move == nil then return end
  color(1,0,0)
  local x,y = crate_to_move.x, crate_to_move.y
  rect('line', left+(x-1)*side, top+(y-1)*side, side, side)
end

crate_to_move = nil

    if (level_state[y][x] == CELL_VACANT or level_state[y][x] == CELL_TARGET) then

    if (level_state[y][x] == CELL_VACANT or level_state[y][x] == CELL_TARGET) and crate_to_move == nil then

    elseif level_state[y][x] == CELL_CRATE or level_state[y][x] == CELL_CRATE_ON_TARGET then
      assert(crate_id[y][x])
      crate_to_move = {x=x, y=y, cid=crate_id[y][x]}
    elseif (level_state[y][x] == CELL_VACANT or level_state[y][x] == CELL_TARGET) and crate_to_move then
      -- TODO: do something here
      crate_to_move = nil

  draw_crate_to_move()