code: func (v *Validate) AddFunction(key string, f Func) error {
  
  	if len(key) == 0 {
  		return errors.New("Function Key cannot be empty")
  	}
  
  	if f == nil {
  		return errors.New("Function cannot be empty")
  	}
  
  	v.validationFuncs[key] = f
  
  	return nil
  }
docs: // AddFunction adds a validation Func to a Validate's map of validators denoted by the key
  // NOTE: if the key already exists, it will get replaced.
  // NOTE: this method is not thread-safe