# Discrete total orders.
interface Discrete
	super Comparable

	redef type OTHER: Discrete

	# The next element.
	fun successor(i: Int): OTHER is abstract

	# The previous element.
	fun predecessor(i: Int): OTHER is abstract

	# The distance between self and d.
	#
	#     assert 10.distance(15)	     ==  5
	#     assert 'Z'.distance('A')	     ==  25
	fun distance(d: OTHER): Int
	do
		var cursor: OTHER
		var stop: OTHER
		if self < d then
			cursor = self
			stop = d
		else if self > d then
			cursor = d
			stop = self
		else
			return 0
		end

		var nb = 0
		while cursor < stop do
			cursor = cursor.successor(1)
			nb += 1
		end
		return nb
	end
end