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

theorem
  A = [: NAT, QC-symbols(A) :]
proof
  consider X being set such that A1: NAT c= X & A = [: NAT, X :] by Def1;
  X <> {} by A1;
  hence A = [: NAT, QC-symbols(A) :] by A1,RELAT_1:160;
end;
