reserve a,b,i,j,k,l,m,n for Nat;

theorem
  for n be Nat, D be set, f be D-valued FinSequence st
    n in dom f holds len(f|n) = n
  proof
    let n be Nat, D be set, f be D-valued FinSequence;
    assume n in dom f; then
    n <= len f by FINSEQ_3:25;
    hence thesis by FINSEQ_1:59;
  end;
