code: func (f *Feature) PropertyInt(key string) (int, error) {
  	if i, ok := (f.Properties[key]).(int); ok {
  		return i, nil
  	}
  
  	if i, ok := (f.Properties[key]).(float64); ok {
  		return int(i), nil
  	}
  
  	return 0, fmt.Errorf("type assertion of `%s` to int failed", key)
  }
docs: // PropertyInt type asserts a property to `int`.