theorem Th109:
  for x, y being Element of VarPoset holds x <= y iff y c= x
proof
  let x, y be Element of VarPoset;
  set V = the set of all varcl A where A is finite Subset of Vars;
  set A0 = the finite Subset of Vars;
  varcl A0 in V;
  then reconsider V as non empty set;
  reconsider a = x, b = y as Element of (InclPoset V) opp;
  x <= y iff ~a >= ~b by YELLOW_7:1;
  hence thesis by YELLOW_1:3;
end;
