theorem Th77:
  <*A1,B1*>(#)<*A2,B2*> = <*A1*A2,B1*B2*>
proof
A1: len <*A2*>=1 by FINSEQ_1:39;
  len <*A1*>=1 by FINSEQ_1:39;
  hence (<*A1,B1*>)(#)(<*A2,B2*>) = (<*A1*>(#)<*A2*>)^(<*B1*>(#)<*B2*>) by A1
,Th74
    .= (<*A1*>(#)<*A2*>)^(<*B1*B2*>) by Th76
    .= <*A1*A2,B1*B2*> by Th76;
end;
