code: func lexValueSequence(l *lexer) stateFn {
  	switch r := l.next(); {
  	case r == eof:
  		return lexStatements
  	case isSpace(r):
  		l.emit(ItemSpace)
  		lexSpace(l)
  	case r == '+':
  		l.emit(ItemADD)
  	case r == '-':
  		l.emit(ItemSUB)
  	case r == 'x':
  		l.emit(ItemTimes)
  	case r == '_':
  		l.emit(ItemBlank)
  	case isDigit(r) || (r == '.' && isDigit(l.peek())):
  		l.backup()
  		lexNumber(l)
  	case isAlpha(r):
  		l.backup()
  		// We might lex invalid items here but this will be caught by the parser.
  		return lexKeywordOrIdentifier
  	default:
  		return l.errorf("unexpected character in series sequence: %q", r)
  	}
  	return lexValueSequence
  }
docs: // lexValueSequence scans a value sequence of a series description.