build.ancestos returns a named list of vectors. Each component corresponds to a node \(x\) of the graph and its vector
is the set of its ancestors including also \(x\).
build.ancestors.per.level returns a named list of vectors. Each component corresponds to a node \(x\)
of the graph and its vector is the set of its ancestors including also \(x\). The nodes are ordered from root (included) to leaves.
build.ancestors.bottom.up a named list of vectors. Each component corresponds to a node \(x\) of the
graph and its vector is the set of its ancestors including also \(x\). The nodes are ordered from leaves to root (included).