from preamble:
"Change Set: UsableSiblingInstanceFix-nk Date: 11 October 2003 Author: Ned Konz
Morph>>usableSiblingInstance was always adding the created sibling to the World, even though many of its callers would turn around and immediately add the result to other morphs or delete them from the World.
This change set makes #usableSiblingInstance not add the newly created Morph to the World.
This makes several uses of #usableSiblingInstance more bug-free and efficient.
"!
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