theorem Th26:
  for p,q being QC-formula of A holds p is closed &
   q is closed iff p => q is closed
proof
  let p,q be QC-formula of A;
A1: p => q = 'not'(p '&' 'not' q) by QC_LANG2:def 2;
  p '&' 'not' q is closed iff p is closed & 'not' q is closed by Th22;
  hence thesis by A1,Th21;
end;
