reserve Omega for set;
reserve X, Y, Z, p,x,y,z for set;
reserve D, E for Subset of Omega;
reserve f for Function;
reserve m,n for Nat;
reserve r,r1 for Real;
reserve seq for Real_Sequence;
reserve F for Field_Subset of X;

theorem
  for A, B being set holds (A in F & B in F implies (A \ B) \/ B in F)
proof
  let A, B be set;
  A \/ B = (A \ B) \/ B by XBOOLE_1:39;
  hence thesis by Th3;
end;
