code: func (f *Feature) PropertyMustInt(key string, def ...int) int {
  	var defaul int
  
  	b, err := f.PropertyInt(key)
  	if err == nil {
  		return b
  	}
  
  	if len(def) > 0 {
  		defaul = def[0]
  	}
  
  	return defaul
  }
docs: // PropertyMustInt guarantees the return of a `bool` (with optional default)
  //
  // useful when you explicitly want a `bool` in a single value return context:
  //     myFunc(f.PropertyMustInt("param1"), f.PropertyMustInt("optional_param", 123))