| 2 | 11. L:Sequent List.
s L.( (s) = 0)
& ( s L.|= s  |= < y2.(M @ N),concl > )
& ( a:Assignment. s L.a | s  a | < y2.(M @ N),concl > ) 12. L:Sequent List.
s L.( (s) = 0)
& ( s L.|= s  |= < M @ N,y1.concl > )
& ( a:Assignment. s L.a | s  a | < M @ N,y1.concl > ) L:Sequent List.
s L.( (s) = 0)
& ( s L.|= s  |= < M @ (y1   y2.N),concl > )
& ( a:Assignment. s L.a | s  a | < M @ (y1   y2.N),concl > ) |