In principle, we could try to make the β we create here the ϝ of the called
wp_let.unlock.
function, but that seems to actually be more work because we could not use
iApply(type_call_iris_[ϝ]ttwith"LFT HE Hna [Hϝ] [Hf'] [Henv Htl Htl† Hx'vl]").4:iExact"Hf'".(* FIXME: Removing the [ ] around Hf' in the spec pattern diverges. *)