WordPress nav, solved: How to list child pages only for the current Parent page

The Problem: You have a 3-tiered navigational hierarchy, like above. If you click on mainPage-a, you only want the 3 child Pages to show. But if you click to dive into Page1, you still want to show the three child Pages of mainPage-a, but also want Page1′s subpages to show, but ONLY those subpages. For [...]