This attribute are lazy to compute the list use get_all_successors
for each needed vertexe.
Warning the cache must be invalidated after add_arc
# Cache of all successors for each vertex.
# This attribute are lazy to compute the list use `get_all_successors` for each needed vertexe.
# Warning the cache must be invalidated after `add_arc`
private var cache_all_successors = new HashMap[V, Set[V]]
lib/graph/digraph.nit:871,2--874,58