code: @RequirePOST
      public User doCreateAccountWithFederatedIdentity(StaplerRequest req, StaplerResponse rsp) throws IOException, ServletException {
          User u = _doCreateAccount(req,rsp,"signupWithFederatedIdentity.jelly");
          if (u!=null)
              ((FederatedIdentity)req.getSession().getAttribute(FEDERATED_IDENTITY_SESSION_KEY)).addTo(u);
          return u;
      }
docs: Creates an account and associates that with the given identity. Used in conjunction
  with {@link #commenceSignup}.