reserve X for non empty BCIStr_1;
reserve d for Element of X;
reserve n,m,k for Nat;
reserve f for sequence of  the carrier of X;

theorem
  (the ExternalDiff of X) "**" <* d *> = d
proof
A1: len<* d *> = 1 by FINSEQ_1:39;
  then
  ex f st f.1 = <* d *>.1 & (for n st 0 <> n & n < len<* d *> holds f.(n +
  1) = (the ExternalDiff of X).(f.n,<* d *>.(n + 1))) & (the ExternalDiff of X)
  "**" <* d *> = f.len<* d *> by FINSOP_1:def 1;
  hence thesis by A1;
end;
