docs: public synchronized VersionedFile get(String fileName, Version version) {
      VersionedFile entry = cache.get(fileName);
      if (entry != null) {
        if (entry.version().equals(version)) {
          // Make a defensive copy since the caller might run further passes on it.
          return entry.copy();
        } else {
          // Aggressively purge to save memory.
          cache.remove(fileName);
        }
      }
      return null;
    }