reserve A for QC-alphabet;
reserve i,j,k,l,m,n for Nat;
reserve a,b,e for set;
reserve t,u,v,w,z for QC-symbol of A;
reserve p,q,r,s for Element of CQC-WFF(A);
reserve x for Element of bound_QC-variables(A);
reserve ll for CQC-variable_list of k,A;
reserve P for QC-pred_symbol of k,A;

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;
