All my previous use of layouts with the Roassal API has been of the form.. ROSomeLayout on: rawview elements. where elements returns both ROElement and ROEdge.

This has never been a problem until I do... ROForceBasedLayout on : rawview elements which fails in doExecute since ROEdge does not understand #center.

This might not have been noticed earlier since the tests send only elements, and the only other use is by ROMondrianExample where ROMondrianFrame>>applyLayout uses 'self nodes' which returns only elements, eg.... ROMondrianFrame>>nodes ^ (elements select: [ :el | el class ~~ ROEdge ]) asArray

So should the caller of a layout _required_ to send only ROElements to a layout (which may clutter user code) or perhaps now that there is ROAbstractComponent, ROView>>elements might return only ROElements, with ROView>>components returning both ROElement and ROEdge ? (except I think you wanted to minimise the dependency of separate ROElement/ROEdge.

Or perhaps ROLayout>>on: might filter out ROEdge(s) ?

cheers -ben