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 Th14:
  g "**" <* d1,d2,d3 *> = g.(g.(d1,d2),d3)
proof
A1: len<* d1,d2 *> = 2 by FINSEQ_1:44;
  thus g "**" <* d1,d2,d3 *> = g "**" (<* d1,d2 *> ^ <* d3 *>) by FINSEQ_1:43
    .= g.(g "**" <* d1,d2 *>,d3) by A1,Th4
    .= g.(g.(d1,d2),d3) by Th12;
end;
