docs: function tween(a, b){
  	var string = []
  	var keys = []
  	var from = []
  	var to = []
  	var cursor = 0
  	var m
  
  	while (m = number.exec(b)) {
  		if (m.index > cursor) string.push(b.slice(cursor, m.index))
  		to.push(Number(m[0]))
  		keys.push(string.length)
  		string.push(null)
  		cursor = number.lastIndex
  	}
  	if (cursor < b.length) string.push(b.slice(cursor))
  
  	while (m = number.exec(a)) from.push(Number(m[0]))
  
  	return function frame(n){
  		var i = keys.length
  		while (i--) string[keys[i]] = from[i] + (to[i] - from[i]) * n
  		return string.join('')
  	}
  }