In case that Gj, 1 ~ j < m, consists of more than one connected
component, Heath and Pemmaraju suggest to use a PQ-tree for every component
and formulate a set of rules of how to merge components F1 and F2, respectively
their corresponding PQ-trees T1 and T2, if F1 and F2 both are adjacent to some
vertex v E Vj+I.