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

theorem Th3: NAT c= QC-symbols(A) & 0 in QC-symbols(A)
proof
    consider X being set such that A1: NAT c= X & A = [: NAT, X :] by Def1;
    thus A2: NAT c= QC-symbols(A) by A1,RELAT_1:160;
    thus 0 in QC-symbols(A) by A2;
end;
