code: func (r *claimResolver) Verifier(iss string) (*oidc.IDTokenVerifier, error) {
  	r.m.Lock()
  	av := r.verifierPerIssuer[iss]
  	if av == nil {
  		// This lazy init should normally be very quick.
  		// TODO: Make this context cancelable.
  		ctx := oidc.ClientContext(context.Background(), r.client)
  		av = newAsyncIDTokenVerifier(ctx, r.config, iss)
  		r.verifierPerIssuer[iss] = av
  	}
  	r.m.Unlock()
  
  	v := av.verifier()
  	if v == nil {
  		return nil, fmt.Errorf("verifier not initialized for issuer: %q", iss)
  	}
  	return v, nil
  }
docs: // Verifier returns either the verifier for the specified issuer, or error.