If by URL you mean the rendered paths, Iβm not sure I understand: systems/sys1/_index.md and systems/sys1.md will both be rendered to yoursite.com/systems/sys1/ (assuming default configs elsewhere) so that URL will not change.
If you really do not want to do that, you could perhaps play around with some .File.Path filtering.
This is possible with the .CurrentSection variable, which would give access for a1.md to all pages in /systems/sys1/. Thereβs more information here: section page variables and methods.
This does require that you use _index.md files. Even though you donβt, Hugo requires them. I donβt know of an option to work around that.
Ha! Itβs working. I hadnβt understood that _index.md defines a section, apparently, and that .CurrentSection.Pages contains the pages of the current section. Thanks, everybody.