What is the test for a plaintiff in a "backward-looking" access to the courts action?

Specifically, in a "backward-looking" access to the courts action,¹ a plaintiff must describe (1) a nonfrivolous underlying claim that was allegedly compromised "to show that the 'arguable' nature of the claim is more than hope"; (2) the official acts that frustrated the litigation of that underlying claim; and (3) a "remedy available under the access claim and presently unique to it" that could not be awarded by bringing a separate action on an existing claim. Christopher v. Harbury, 536 U.S. at 416.