In nbpunch, support reading chads from multiple source notebooks
-s) option supports only a single notebook as source for chads.
Nevertheless, it seems to work when this option is used more than once,
since the source notebook is read immediately upon parsing the argument.
But the stored file name is overwritten, and only the last one sticks;
that stored value seems to be used only as a boolean.
It should not be hard to provide better support for multiple such source notebooks.
That would mean that the
source options accumulates its parameters.
And probably also that loading of the source chads is delayed until all options have been parsed.