Network of the most frequent SPRs from Breassort.Each node represents the set of trees for the corresponding segment. Edge colours correspond to different types of SPR operations, as shown. Edges point from a segment that proposes the branch swap, to the one that needs to be modified (ending in filled circle). For example, the orange edge going from NA to NS depicts the following operation: cutting the branch leading to A/HongKong/1073/99 H9N2 and reconnecting it to the branch ending in A/HongKong/1774/99 H3N2. The NS tree is the one being modified, and the NA tree proposes this modification.