Comment #6 on issue 576 by tudor.gi...@gmail.com: Border of children depends on another shape in a complex shape http://code.google.com/p/moose-technology/issues/detail?id=576
Definitely, you need the shape as key!
However, forming an anonymous array as key is not nice. We should have a two level structure. First, you look for the shape and get a ElementShapeProperties. Then you look for the specific properties inside.