theorem
  for B,C st B in Class PropRel Q & C in Class PropRel Q for p,q holds
  'not' p in C & 'not' q in C & p in B implies q in B
proof
  let B,C such that
A1: B in Class PropRel Q & C in Class PropRel Q;
  let p,q;
  'not'('not' p) = p & 'not'('not' q) =q;
  hence thesis by A1,Th11;
end;
