# Temporary remove `e`.
	#
	# Children of `e` are left untouched in the tree.
	# This make the tree inconstant until `e` is added back.
	private fun detach(e: E)
	do
		var old_parent = parents.get_or_null(e)
		if old_parent != null then
			var subs = sub[old_parent]
			subs.remove(e)
			if subs.is_empty then
				# remove the sub when all children are detached
				# so that `==` and `hash` are sane
				# Otherwise an empty array will be considered
				# differently than no array.
				sub.keys.remove(old_parent)
			end
		else if roots.has(e) then
			roots.remove(e)
		end
	end