A modular decomposition tree is the rooted tree whose nodes are the strong modules of a graph,
ordered by inclusion. Its root is the full vertex set,
its leaves are the singleton sets, and the children
of a node are its maximal proper strong submodules.
An internal node is parallel when the quotient on its children is an empty graph, series when it is a complete graph,
and prime otherwise. A cograph is characterized by having
no prime nodes.
