code: func (p *Parser) parseExactValue() (sets.String, error) {
  	s := sets.NewString()
  	tok, lit := p.lookahead(Values)
  	if tok == EndOfStringToken || tok == CommaToken {
  		s.Insert("")
  		return s, nil
  	}
  	tok, lit = p.consume(Values)
  	if tok == IdentifierToken {
  		s.Insert(lit)
  		return s, nil
  	}
  	return nil, fmt.Errorf("found '%s', expected: identifier", lit)
  }
docs: // parseExactValue parses the only value for exact match style