mutations

const add = (list, item) => {
	return […list, item]
}

const remove = (list, index) => {
	return [
		…list.slice(0,index),
		…list.slice(index+1)
	]
}

const increment = (list, index) => {
	return [
		…list.slice(0,index),
		list[index] + 1,
		…list.slice(index+1)
	]
}