code: func (l *ProviderList) Provider(name string) (common.Provider, error) {
  
  	// panic on nil
  	if l == nil {
  		panic(common.PrefixForErrors + "No providers have been initialised.  Make sure you have called gomniauth.WithProviders(...).")
  	}
  
  	for _, provider := range l.providers {
  		if provider.Name() == name {
  			return provider, nil
  		}
  	}
  
  	return nil, &common.MissingProviderError{ProviderName: name}
  }
docs: // Provider gets a provider by name, or returns a common.MissingProviderError
  // if no provider with that name is registered.