# Combine the subsets of `e` and `f`
	# ENSURE: `in_same_subset(e,f)`
	fun union(e,f:E)
	do
		var ne = find(e)
		var nf = find(f)
		if ne == nf then return

		# merge them using *union by rank*
		# attach the smaller tree to the root of the deeper tree
		var er = ne.rank
		var fr = nf.rank
		if er < fr then
			ne.parent = nf
			nodes[e] = nf
		else
			nf.parent = ne
			nodes[f] = ne
			if er == fr then
				# The only case where the deep is increased is when both are equals
				ne.rank = er+1
			end
		end
		number_of_subsets -= 1
	end