T:(Id), to,from:(|T|(IdLnk List)), f:(Edge(T)).
  bi-tree(T;to;from)
  
  spanner(f;T;to;from)  (i:|T|. spanner-root(f;T;to;from;i))

1

1. T : Id 2. to : |T|(IdLnk List) 3. from : |T|(IdLnk List) 4. f : Edge(T) 5. bi-tree(T;to;from) 6. bi-graph(T;to;from) 7. spanner(f;T;to;from)   i:|T|. spanner-root(f;T;to;from;i)

34 steps

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