Figure 4.

COC Procedure. An instance of the COC problem. In the gene tree G, where s(a₁) = s(a₂) = A, s(b₁) = s(b₂) = B and s(c) = C, extract and copy the subtree rooted at × to get the subtrees G^A(x) and G^B(x). Remove b₁ and b₂ from G^A(x) and a₁ and a₂ from G^B(x). Join G^A(x) and G^B(x) under a common root and replace G(x) by the new subtree in the gene tree G'.