docs: @JsonCreator
    public static synchronized Symbol from(@JsonProperty("string") final String string) {
      final WeakReference<Symbol> ref = symbols.get(checkNotNull(string));
  
      if (ref != null) {
        final Symbol sym = ref.get();
        if (sym != null) {
          return sym;
        }
      }
  
      final Symbol sym = new Symbol(string);
      symbols.put(string, new WeakReference<Symbol>(sym));
      return sym;
    }