Reference barriers (MIT/GNU Scheme 10.1.4)

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#### 10.7.3 Reference barriers

The garbage collector may break an ephemeron if it can prove that the key is not strongly reachable. To ensure that it does not do so before a certain point in a program, the program can invoke a reference barrier on the key by calling the `reference-barrier` procedure, which guarantees that even if the program does not use the key, it will be considered strongly reachable until after `reference-barrier` returns.

procedure: reference-barrier object

  Guarantee that object is strongly reachable until after `reference-barrier` returns.