theorem Th51:
  C is I_Lattice implies a => b = "\/"({c: a"/\"c [= b}, C)
proof
  set X = {a9: a"/\"a9 [= b};
  assume
A1: C is I_Lattice;
  then a"/\"(a=>b) [= b by FILTER_0:def 7;
  then
A2: a=>b in X;
  X is_less_than a=>b
  proof
    let c;
    assume c in X;
    then ex a9 st c = a9 & a"/\"a9 [= b;
    hence c [= a=>b by A1,FILTER_0:def 7;
  end;
  hence thesis by A2,Th40;
end;
