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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _set_default_versions(self, config):
 """Retrieve pre-computed version information for expensive to retrieve versions. Starting up GATK takes a lot of resources so we do it once at start of analysis. """

        out = []
        for name in ["gatk", "gatk4", "picard", "mutect"]:
            v = tz.get_in(["resources", name, "version"], config)
            if not v:
                try:
                    v = programs.get_version(name, config=config)
                except KeyError:
                    v = None
            out.append(v)
        self._gatk_version, self._gatk4_version, self._picard_version, self._mutect_version = out