simp only [WellFormed, PWellFormed] at wf
I : Type u₀
C : IContainer' I
i : I
x✝ : M C i
m : Container.M (toContainer C)
wf : WellFormed i m
⊢ i = Idx C (Container.M.destruct ↑{ val := m, property := wf }).fst
I : Type u₀
C : IContainer' I
i : I
x✝ : M C i
m : Container.M (toContainer C)
wf : ↑(pgfp (WellFormedF C)) ⊥ i m
⊢ i = Idx C (Container.M.destruct ↑{ val := m, property := wf }).fst