The NNI adjacency graph. (a) The tree is obtained from G by pruning and regrafting subtree G
to the root of G. The vertex x ∈ V(G) is suppressed, and the new vertex above root in is named x. (b) Two NNI operations NNI
(z') and NNI
(z) produce left-child G'
and right-child G'
of G' in .