checkpoint=klass()
function checkpoint:init(board,card)
 self.card=card
 self.slots={}
 self.wells={}
 for s=1,#board.slots do
  local cnt={}
  for c in all(board.slots[s].contents) do
   add(cnt,c)
  end
  self.slots[s]=cnt
 end
 for w=1,#board.wells do
  local cnt={}
  for c in all(board.wells[w].contents) do
   add(cnt,c)
  end
  self.wells[w]=cnt
 end
end

function checkpoint:apply(board)
 for s=1,#board.slots do
  while #board.slots[s].contents>0 do
   deli(board.slots[s].contents)
  end
  for i in all(self.slots[s]) do
   add(board.slots[s].contents,i)
  end
 end
 for w=1,#board.wells do
  while #board.wells[w].contents>0 do
   deli(board.wells[w].contents)
  end
  for i in all(self.wells[w]) do
   add(board.wells[w].contents,i)
  end
 end
end