docs: public static List<Class<?>> findAllImplementations(Class<?> restrictionClass) {
      if(restrictionClass == null) {
        return Collections.emptyList();
      }
      if(!contains(restrictionClass)) {
        ELKIServiceLoader.load(restrictionClass);
        ELKIServiceScanner.load(restrictionClass);
      }
      Entry e = data.get(restrictionClass);
      if(e == null) {
        return Collections.emptyList();
      }
      // Start loading classes:
      ArrayList<Class<?>> ret = new ArrayList<>(e.len);
      for(int pos = 0; pos < e.len; pos++) {
        Class<?> c = e.clazzes[pos];
        if(c == null) {
          c = tryLoadClass(e.names[pos]);
          if(c == null) {
            LOG.warning("Failed to load class " + e.names[pos] + " for interface " + restrictionClass.getName());
            c = FAILED_LOAD;
          }
          e.clazzes[pos] = c;
        }
        if(c == FAILED_LOAD) {
          continue;
        }
        // Linear scan, but cheap enough.
        if(!ret.contains(c)) {
          ret.add(c);
        }
      }
      return ret;
    }