
theorem Th33:
  for L being satisfying_Sheffer_1 satisfying_Sheffer_2
  satisfying_Sheffer_3 properly_defined non empty ShefferOrthoLattStr holds L
  is join-commutative
proof
  let L be satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3
  properly_defined non empty ShefferOrthoLattStr;
  let x, y be Element of L;
  x "\/" y = x" | (y | y) by Def12
    .= (y | y) | (x | x) by Th31
    .= y "\/" x by Def12;
  hence thesis;
end;
