Let x0 of type ι be given.
Let x1 of type ι be given.
Let x2 of type ι → ι be given.
Let x3 of type ι be given.
Assume H1: x3 ∈ x0.
Apply explicit_Nats_primrec_base with
x0,
x1,
x2,
x2 x3,
λ x4 x5 . x2 x5.
The subproof is completed by applying H0.