The following method is used for controling the scope of edge definition.
However, the problem is that if the target node is not found locally, the
search is restarted globally.

localLookupNode: aModel

^ self root 
    nodeWith: aModel    
    ifAbsent: [ self root 
        firstSubnodeThat: [:aNode | aNode model = aModel] 
        ifAbsent: [ self root root
           firstSubnodeThat: [:aNode | aNode model = aModel] ] ]

This should not happen. So, the last ifAbsent: should be removed. I tried
to remove it, but it looks like two tests related to adding edges through
interaction break. I do not understand why, so I left the original code for
now.