code: func (sd structDef) needFmt() bool {
  	// array type min items and max items
  	if sd.T.MinItems > 0 || sd.T.MaxItems > 0 {
  		return true
  	}
  
  	// unique items
  	for _, f := range sd.Fields {
  		if f.UniqueItems {
  			return true
  		}
  	}
  	return false
  }
docs: // true if this struct need to import 'fmt' package
  // It is required by validation code,
  // because validation error will need `fmt` to build error message