theorem
  FinJoin(B,f) [= u implies for x st x in B holds f.x [= u
proof
  assume
A1: FinJoin(B,f) [= u;
  let x;
  assume x in B;
  then f.x [= FinJoin(B,f) by Th28;
  hence thesis by A1,LATTICES:7;
end;
