reserve S for 1-sorted,
  i for Element of NAT,
  p for FinSequence,
  X for set;

theorem
  for p being FinSequence of S st i in dom p holds p.i in S
proof
  let p be FinSequence of S;
  assume i in dom p;
  hence p.i in the carrier of S by FINSEQ_2:11;
end;
