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 = 2 implies g "**" F = g.(F.1,F.2)
proof
  assume
A1: len F = 2;
  then F = <* F.1,F.2 *> by FINSEQ_1:44
    .= <* F/.1,F.2 *> by A1,FINSEQ_4:15
    .= <* F/.1,F/.2 *> by A1,FINSEQ_4:15;
  hence g "**" F = g.(F/.1,F/.2) by Th12
    .= g.(F.1,F/.2) by A1,FINSEQ_4:15
    .= g.(F.1,F.2) by A1,FINSEQ_4:15;
end;
