# Split `self` on the first occurence of `pattern`
	#
	#     assert "hello".split_once_on('l') == ["he", "lo"]
	#     assert "a, b, c, d, e".split_once_on(", ") == ["a", "b, c, d, e"]
	fun split_once_on(pattern: Pattern): Array[SELFTYPE]
	do
		var m = pattern.search_in(self, 0)
		var res = new Array[SELFTYPE]
		if m == null then
			res.add self
		else
			res.add substring(0, m.from)
			res.add substring_from(m.after)
		end
		return res
	end