/**
   * Adds {@code delta} to the value currently associated with {@code key}, and returns the old
   * value.
   */
  @CanIgnoreReturnValue
  public long getAndAdd(K key, long delta) {
    return getAndAccumulate(key, delta, Long::sum);
  }

  /**
   * Updates the value currently associated with {@code key} with the specified function, and