Part 4 of "Event-Driven" Tutorial on the WD-ABAP Component WDR_TEST_UI_ELEMENTS.

(...in which it is learned how many different ways the term "element" can be used to mean entirely different things.)
This post resumes the "experiential" or "event-driven" tutorial on how to use the SAP-delivered WD-ABAP component WDR_TEST_UI_ELEMENTS as a pattern for a generic component that displays a detail view from a tree view from a tray view. In particular, this post will show how the debugger was kind enough to let me stumble almost immediately onto the correct way to change WDR_TEST_UI_ELEMENTS so that it displays trees in the right-hand-side tree-view even when one has not defined "runtime_classes" for the custom links one has placed in the left-hand-side custom trays.