sclass symbol_Tester {
  S text;
  
  *(S *text) {}
  
  public int hashCode() { ret main.hashCode(text); }
  public bool equals(O o) {
    ret o instanceof Symbol && eq(text, o/Symbol.text);
  }
}

static WeakHashMap<Symbol, Bool> symbol_map = newWeakHashMap();

static Symbol symbol(S s) {
  if (s == null) null;
  synchronized(symbol_map) {
    // This is a trick that works because of how WeakHashMap compares keys.
    O e = call(symbol_map, 'getEntry, symbol_Tester(s));
    Symbol sym = e == null ? null : ((WeakReference<Symbol>) e).get();
    if (sym == null)
      symbol_map.put(sym = Symbol(s, false), Bool.TRUE);
    ret sym;
  }
}