reserve A for QC-alphabet;
reserve k,n,m for Nat;

theorem Th6:
  QC-pred_symbols(A) c= [: NAT, QC-symbols(A) :]
proof
  let y be object;
  assume y in QC-pred_symbols(A);
  then consider k being Nat, x being QC-symbol of A such that
A1:  y = [k, x] & 7 <= k;
   k in NAT by ORDINAL1:def 12;
  hence thesis by ZFMISC_1:def 2,A1;
end;
