theorem
  for i being Nat for D being set, P being FinSequence of D st 1 <= i &
  i <= len P holds P/.i = P.i
proof
  let i be Nat;
  let D be set, P be FinSequence of D;
  assume 1 <= i & i <= len P;
  then i in dom P by FINSEQ_3:25;
  hence thesis by PARTFUN1:def 6;
end;
