theorem
  a,b9 // a,b99 & a,b // a,b99 implies b=b9 or b=b99 or b9=b99
proof
  assume
A1: a,b9 // a,b99 & a,b // a,b99;
  now
    assume b9<>b99 & b<>b99;
    then b9,b // b99,b99 by A1,Def1;
    hence thesis by Th6;
  end;
  hence thesis;
end;
