
theorem
  for MySigmaField, A1, A2 being set st
    MySigmaField={{},{1,2,3,4}} & A1 in MySigmaField & A2 in MySigmaField
      holds
   A1 /\ A2 in MySigmaField
proof
  let MySigmaField,A1,A2 be set;
  assume A0: MySigmaField={{},{1,2,3,4}} &
    A1 in MySigmaField & A2 in MySigmaField; then
  (A1 = {} or A1 = {1,2,3,4}) & (A2 = {} or A2 = {1,2,3,4}) by TARSKI:def 2;
  hence thesis by A0;
end;
