The following is a sealer-unsealer pattern in the language E. This is Bill Frantz’s version of Kevin Reid’s version.

def makeBrandPair(){
var temp := null
def sealer(val){
  def box(){
    temp := [val, box]
  }; return box;}
  def unsealer(box){
    temp:= null
    box()
    def temp2 := temp
    temp := null
    if(temp2[1] == box){
      return temp2[0]
    }else throw
  }
  return [sealer, unsealer]
}

Indeed there is an application of this function in the makeMint code in the article above. Here is the more elaborate official sealer.

And another!