According to the tutorial, it seems that every option has a name and can be specified as a regular option. Is it possible, to enforce an option to be only legal if specified as a positional option, and not show up in the help message as a regular option?
This exact use-case is described later in the tutorial. You have to create two different options_description objects, one for the visible, an other for the hidden options, then mix them together in a third options_description object. When printing the help, describe only the visible one.