Incremental expansion of nodes in graph-pane

Hi

I have a rather large display of a DAG in a graph-pane.  It would help if I could start with only the root(s) shown in the pane and then expand them at will.

The current behavior is that I get the whole DAG displayed and if I manually collapse the roots any re-expansion will redisplay the *whole* subgraph.  There must be a way to control this.  Any quick idea?

thanks

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Marco Antoniotti