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

theorem
  ( p <=> q ) => ( p => q ) in TAUT(A)
proof
  ( p <=> q ) = ( p => q ) '&' ( q => p ) by QC_LANG2:def 4;
  hence thesis by Th19;
end;
