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2. More general parallel tree contraction: Register allocation and broadcasting in a tree
   Krzysztof Diks and Torben Hagerup
   In: Graph-theoretic concepts in computer science (WG-96) : 22nd international workshop, Cadenabbia, Italy,Graph-theoretic concepts in computer science (WG-96) : 22nd international workshop, Cadenabbia, Italy, 1996, 126-140
   
1. More general parallel tree contraction: Register allocation and broadcasting in a tree
   Krzysztof Diks and Torben Hagerup
   Theoretical Computer Science 203 (1): 3-29, 1998