Let x0 of type ι be given.
Let x1 of type ι → ι be given.
Let x2 of type ι be given.
Assume H0: x2 ∈ x0.
Let x3 of type ι be given.
Assume H1: x3 ∈ x1 x2.
Apply famunionI with
x0,
λ x4 . {setsum x4 x5|x5 ∈ x1 x4},
x2,
setsum x2 x3 leaving 2 subgoals.
The subproof is completed by applying H0.
Apply ReplI with
x1 x2,
setsum x2,
x3.
The subproof is completed by applying H1.