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Definition: A tree is a directed graph such that there is exactly one node, the root, that has indegree zero, every other node has indegree one, and every node can be reached from the root.

T is tree : <=>
   T is directed graph /\ 
   (exists r in V: indeg(r) = 0 /\ 
      forall x in V-{r}:
         indeg(x) = 1 /\ 
         x is reachable from r in T) where V = T0.
root(T) := (such r in V: indeg(r) = 0) where V = T0.

Author: Wolfgang Schreiner
Last Modification: January 26, 2000

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