Readable code example, i think, is:
```ruby
if (@output_per_tag || @tag_prefix) && (!@output_per_tag || !@tag_prefix)
  raise Fluent::ConfigError, 'Specify both of output_per_tag and tag_prefix'
end
@tag_prefix_string = @tag_prefix + '.' if @output_per_tag
```
XOR is just for this case, but we should not use XOR.