code: func validUTF8(c1, c2 uint32) bool {
  	switch {
  	case c1 < 0x80:
  		// 1-byte, must be followed by 1-byte or first of multi-byte
  		return c2 < 0x80 || 0xc0 <= c2 && c2 < 0xf8
  	case c1 < 0xc0:
  		// continuation byte, can be followed by nearly anything
  		return c2 < 0xf8
  	case c1 < 0xf8:
  		// first of multi-byte, must be followed by continuation byte
  		return 0x80 <= c2 && c2 < 0xc0
  	}
  	return false
  }
docs: // validUTF8 reports whether the byte pair can appear in a
  // valid sequence of UTF-8-encoded code points.