reserve A for QC-alphabet;
reserve p, q, r, s for Element of CQC-WFF(A);

theorem
  ( p => q ) => (( q => p ) => ( p <=> q )) in TAUT(A)
proof
  ( p => q ) => (( q => p ) => (( p => q ) '&' ( q => p ))) in TAUT(A) by Th28;
  hence thesis by QC_LANG2:def 4;
end;
