theorem
  <j> * <j> = -1
proof
A1: <j> * <j> = [* 0*0-0*0-1*1-0*0, 0*0+0*0+0*0-0*1,
  0*1+0*1+0*0-0*0, 0*0+0*0+0*1-1*0 *] by Def9
    .= [*-1,0,0,0*];
  [*-1,0,0,0*] = [*-jj,zz*] by Lm3;
  hence thesis by A1,ARYTM_0:def 5;
end;
