# Return the corresponding digit character
	# If 0 <= `self` <= 9, return the corresponding character.
	#
	#     assert 5.to_c    == '5'
	#
	# If 10 <= `self` <= 36, return the corresponding letter [a..z].
	#
	#     assert 15.to_c   == 'f'
	fun to_c: Char
	do
		assert self >= 0 and self <= 36 # TODO plan for this
		if self < 10 then
			return (self + '0'.code_point).code_point
		else
			return (self - 10 + 'a'.code_point).code_point
		end
	end