checkArgument(length >= 0, "length (%s) may not be negative", length);
      long maxLength = this.length - offset;
      return maxLength <= 0
          ? ByteSource.empty()
          : ByteSource.this.slice(this.offset + offset, Math.min(length, maxLength));
    }

    @Override
    public boolean isEmpty() throws IOException {
      return length == 0 || super.isEmpty();