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

theorem
  X |- p => (q => r) & X|- p => q implies X |- p => r
proof
  assume X|- p => (q => r);
  then X |- (p => q) => (p =>r) by Th67;
  hence thesis by CQC_THE1:55;
end;
