reserve x,z for set;
reserve k for Element of NAT;
reserve D for non empty set;
reserve X for set;
reserve p,r for relation;
reserve a,a1,a2,b for FinSequence;

theorem
  {a} is relation
proof
  for z being object st z in {a} holds z is FinSequence by TARSKI:def 1;
  then reconsider X ={a} as FinSequence-membered set by FINSEQ_1:def 19;
  X is with_common_domain;
 hence thesis;
end;
