Let x0 of type ι be given.
Let x1 of type ι be given.
Apply set_ext with
binunion x0 x1,
binunion x1 x0 leaving 2 subgoals.
The subproof is completed by applying binunion_com_Subq with x0, x1.
The subproof is completed by applying binunion_com_Subq with x1, x0.