theorem Th28:
  SepVar 'not' p = 'not' SepVar p
proof
  reconsider FP = (SepFunc(A)).p as Function of
   [:QC-symbols(A),Funcs(bound_QC-variables(A), bound_QC-variables(A)):],
    CQC-WFF(A);
  thus SepVar 'not' p =(NEGATIVE FP).[index ('not' p),id bound_QC-variables(A)]
  by Def7
    .= (NEGATIVE FP).[index p,(id bound_QC-variables(A))] by Th23
    .= 'not' SepVar p by Def2;
end;
