code: func attributeSubstringSelector(key, val string) Selector {
  	return attributeSelector(key,
  		func(s string) bool {
  			if strings.TrimSpace(s) == "" {
  				return false
  			}
  			return strings.Contains(s, val)
  		})
  }
docs: // attributeSubstringSelector returns a Selector that matches nodes where
  // the attribute named key contains val.