docs: static <T> SettableSupplier<CompletableFuture<T>> settableSupplierOf(Supplier<CompletableFuture<T>> supplier) {
      return new SettableSupplier<CompletableFuture<T>>() {
        volatile boolean called;
        volatile CompletableFuture<T> value;
  
        @Override
        public CompletableFuture<T> get() {
          if (!called && value != null) {
            called = true;
            return value;
          } else
            return supplier.get();
        }
  
        @Override
        public void set(CompletableFuture<T> value) {
          called = false;
          this.value = value;
        }
      };
    }