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 Th12:
  g "**" <* d1,d2 *> = g.(d1,d2)
proof
A1: len<* d1 *> = 1 by FINSEQ_1:39;
  thus g "**" <* d1,d2 *> = g "**" (<* d1 *> ^ <* d2 *>) by FINSEQ_1:def 9
    .= g.(g "**" <* d1 *>,d2) by A1,Th4
    .= g.(d1,d2) by Lm4;
end;
