Use f_equal.
Apply famunion_ext to the current goal.
Let k be given.
Assume Hk.
Use f_equal.
We will
prove ∀w ∈ SNoS_ (SNoLev x), g w = h w.
An exact proof term for the current goal is Hgh.
An exact proof term for the current goal is omega_nat_p k Hk.