Let X, Tx and n be given.
We prove the intermediate
claim HnO:
n ∈ ω.
Apply andI to the current goal.
An exact proof term for the current goal is HTx.
We use n to witness the existential quantifier.
Apply andI to the current goal.
An exact proof term for the current goal is HnO.
An exact proof term for the current goal is Hprop.
∎