| 1 | 11. L: Sequent List 12. s L.( (s) = 0) 13. s L.|= s  |= < y2.(M @ N),concl > 14. a:Assignment. s L.a | s  a | < y2.(M @ N),concl > 15. L1: Sequent List 16. s L1.( (s) = 0) 17. s L1.|= s  |= < M @ N,y1.concl > 18. a:Assignment. s L1.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 > ) |