theorem Th39:
  for a9,b9 being Element of L.: st a9 [= b9 & a = a9 & b = b9 holds b [= a
proof
  let a9,b9 be Element of L.:;
  assume that
A1: a9 [= b9 and
A2: a = a9 & b = b9;
  a9 "\/" b9 = b9 by A1;
  then b "/\" a = b by A2,Th37;
  hence thesis by LATTICES:4;
end;
