code: func permWithAttr(attr, val string) *Constraint {
  	c := &Constraint{
  		Permanode: &PermanodeConstraint{
  			Attr:       attr,
  			SkipHidden: true,
  		},
  	}
  	if val == "" {
  		c.Permanode.ValueMatches = &StringConstraint{Empty: true}
  	} else {
  		c.Permanode.Value = val
  	}
  	return c
  }
docs: // Helpers