Let x0 of type ι be given.
Apply mul_SNo_com with
0,
x0,
λ x1 x2 . x2 = 0 leaving 3 subgoals.
The subproof is completed by applying SNo_0.
The subproof is completed by applying H0.
Apply mul_SNo_zeroR with
x0.
The subproof is completed by applying H0.