theorem
{y} is (SQ,D)-derivable implies ex mm st y is (mm,SQ,D)-derivable
proof
set Q=S-sequents; reconsider Seqts=SQ as Element of bool Q
by Def3; {y} is (Seqts,D)-derivable implies ex mm st
y is (mm,Seqts,D)-derivable by Lm8; hence thesis;
end;
