# Splits `self` in two parts at the first occurence of `b`
	#
	#     var a = "String is string".to_bytes.split_once_on(u's')
	#     assert a[0].hexdigest == "537472696E672069"
	#     assert a[1].hexdigest == "20737472696E67"
	fun split_once_on(b: BytePattern): Array[Bytes] do
		var spl = b.first_index_in(self)
		if spl == -1 then return [clone]
		var ret = new Array[Bytes].with_capacity(2)
		ret.add(slice(0, spl))
		ret.add(slice_from(spl + b.pattern_length))
		return ret
	end