theorem
  for p,q being QC-formula of A holds p is closed & q is closed iff p 'or' q
  is closed
proof
  let p,q be QC-formula of A;
A1: p 'or' q = 'not'('not' p '&' 'not' q) by QC_LANG2:def 3;
  'not' p '&' 'not' q is closed iff 'not' p is closed & 'not' q is closed
  by Th22;
  hence thesis by A1,Th21;
end;
