Let x0 of type ι be given.
Let x1 of type ι be given.
Let x2 of type ι be given.
Assume H0: x2 ∈ x0.
Apply binunionI1 with
{Inj0 x3|x3 ∈ x0},
{Inj1 x3|x3 ∈ x1},
Inj0 x2.
Apply ReplI with
x0,
Inj0,
x2.
The subproof is completed by applying H0.