theorem
  p is negative implies ex q st p = 'not' q
proof
  assume p is negative;
  then consider r being Element of QC-WFF(A) such that
A1: p = 'not' r by QC_LANG1:def 19;
  r is Element of CQC-WFF(A) by A1,CQC_LANG:8;
  hence thesis by A1;
end;
