reserve i,j,k,n for Nat;
reserve D for non empty set,
  p for Element of D,
  f,g for FinSequence of D;

theorem
  for f, g being FinSequence holds (f^g)|(len f) = f
proof
  let f, g be FinSequence;
  thus f = f|len f by FINSEQ_2:20
    .= (f^g)|(len f) by Th22;
end;
