# Build an ordered tree with from `concerns`
	fun concerns_tree(mconcerns: Collection[MConcern]): ConcernsTree do
		var seen = new HashSet[MConcern]
		var res = new ConcernsTree

		var todo = new Array[MConcern]
		todo.add_all mconcerns

		while not todo.is_empty do
			var c = todo.pop
			if seen.has(c) then continue
			var pc = c.parent_concern
			if pc == null then
				res.add(null, c)
			else
				res.add(pc, c)
				todo.add(pc)
			end
			seen.add(c)
		end

		return res
	end