reserve x,y,y1,y2 for set,
  D for non empty set,
  d,d1,d2,d3 for Element of D,
  F,G,H,H1,H2 for FinSequence of D,
  f,f1,f2 for sequence of D,
  g for BinOp of D,
  k,n,i,l for Nat,
  P for Permutation of dom F;

theorem
  len F >= 1 implies ex f st f.1 = F.1 &
 (for n being Nat st 0 <> n & n < len F
  holds f.(n + 1) = g.(f.n,F.(n + 1))) & g "**" F = f.(len F) by Def1;
