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

theorem Th4:
  QC-variables(A) c= [: NAT, QC-symbols(A) :]
proof
  { 6 } c= NAT & NAT c= QC-symbols(A) by Th3,ZFMISC_1:31;
  then
A1: [: { 6 }, NAT :] c= [: NAT, QC-symbols(A) :] by ZFMISC_1:96;
  {4, 5} c= NAT & QC-symbols(A) c= QC-symbols(A) by ZFMISC_1:32;
  then [: {4,5}, QC-symbols(A) :] c= [: NAT, QC-symbols(A) :] by ZFMISC_1:96;
  hence thesis by A1,XBOOLE_1:8;
end;
