# Build `self` from a set of formal `concepts`.
	init from_concepts(concepts: Set[FormalConcept[O, A]]) do
		for c in concepts do
			add_node c
		end
		for c1 in concepts do
			for c2 in concepts do
				if c1 == c2 then continue
				if not is_lower_neighbour(c1, c2, concepts) then continue
				add_edge(c2, c1)
			end
		end
	end