reserve i,k for Nat;
reserve A for QC-alphabet;
reserve x for bound_QC-variable of A;
reserve a for free_QC-variable of A;
reserve p,q for Element of QC-WFF(A);
reserve l for FinSequence of QC-variables(A);
reserve P,Q for QC-pred_symbol of A;
reserve V for non empty Subset of QC-variables(A);
reserve s,t for QC-symbol of A;

theorem Th70:
  Fixed(p => q) = Fixed p \/ Fixed q
proof
  p => q = 'not'(p '&' 'not' q) by QC_LANG2:def 2;
  hence Fixed(p => q) = Fixed(p '&' 'not' q) by Th39
    .= Fixed p \/ Fixed 'not' q by Th42
    .= Fixed p \/ Fixed q by Th39;
end;
