theorem Th2:
  QC-symbols(Al) c= QC-symbols(Al2)
proof
  Al c= Al2 & Al = [: NAT, QC-symbols(Al) :] &
  Al2 = [:NAT,QC-symbols(Al2) :] by Def1, QC_LANG1:5;
  hence QC-symbols(Al) c= QC-symbols(Al2) by ZFMISC_1:115;
end;
