code: func prefixLen(a, b []byte) int {
  	var i, length = 0, len(a)
  	if len(b) < length {
  		length = len(b)
  	}
  	for ; i < length; i++ {
  		if a[i] != b[i] {
  			break
  		}
  	}
  	return i
  }
docs: // prefixLen returns the length of the common prefix of a and b.