theorem
  <j> * <k> = <i>
proof
  <j> * <k> = [* 0*0-0*0-1*0-0*1, 0*0+0*0+1*1-0*0,
  0*0+0*1+0*0-1*0, 0*1+0*0+0*0-1*0 *] by Def9
    .= [*0,1,0,0*];
  hence thesis by Lm2,Lm3;
end;
