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Perpetual Tokens

In other words, a token that is guaranteed to exist forever, unless it gets garbage-collected! 

    {
      val isPerpetual = {(b: Box) =>
        b.propositionBytes == SELF.propositionBytes && b.tokens == SELF.tokens
      }

      sigmaProp(OUTPUTS.exists(isPerpetual))
    }
(Actually, this is a perpetual collection of tokens (possibly of size zero). But if you protect a singleton token with this script, then it guarantees the token will never be destroyed other than by garbage collection.)

See this thread for full discussion.

Multi-stage#

In multi-stage protocols, one script refers to the script of the next stage (example in script1, we have the statement hash(OUTPUTS(0).propositionBytes) == script2Hash).

But suppose we also want the script2 to refer back to script1, (example we want hash(OUTPUTS(0).propositionBytes) == script1Hash), then its a cyclic reference.

One solution is to store script1Hash in the register of the box containing script2. Additionally, script1 is modified to ensure that the corresponding register of any box containing script2 equals hash(SELF.propositionBytes).

EDIT: While the "vanilla" perpetual token is interesting, the more powerful one is the "max-once-per-block-use" perpetual token, which should be a separate design pattern.