* given ModelAction must synchronize. This function only returns a non-empty
* result when it can locate a release sequence head with certainty. Otherwise,
* it may mark the internal state of the ModelChecker so that it will handle
* given ModelAction must synchronize. This function only returns a non-empty
* result when it can locate a release sequence head with certainty. Otherwise,
* it may mark the internal state of the ModelChecker so that it will handle