# Get the `n`th expression in this match
	#
	# `n == 0` returns this match, and a greater `n` returns the corresponding
	# subexpression.
	#
	# Require: `n >= 0 and n <= subs.length`
	#
	# ~~~
	# var re = "c (d e+) f".to_re
	# var match = "a b c d eee f g".search(re)
	# assert match[0].to_s == "c d eee f"
	# assert match[1].to_s == "d eee"
	# ~~~
	fun [](n: Int): nullable Match do
		if n == 0 then return self
		assert n > 0 and n <= subs.length
		return subs[n-1]
	end