code: func (p *Parser) Selector() bool {
  
  	if p.PeekByte() != '{' {
  		return false
  	}
  	p.Byte()
  
  	i := p.ev.Level()
  
  	p.ev.Add(TypeSelector)
  	p.ev.Inc()
  
  	p.Space()
  	p.Expression()
  	p.Space()
  
  	if p.PeekByte() != '}' {
  		return false // error
  	}
  	p.Byte()
  
  	// Level before and after is the same
  	p.ev.SetLevel(i)
  	return true
  }
docs: // Selector ::= '{' expression? '}'