Let a be given.
We prove the intermediate
claim HaEq:
a = 0.
An
exact proof term for the current goal is
(SingE 0 a Ha0).
rewrite the current goal using HaEq (from left to right).
We prove the intermediate
claim H0O:
0 ∈ ω.
rewrite the current goal using Hdef (from left to right).
∎