reserve a,b,c,d,x,y,w,z,x1,x2,x3,x4 , X for set;
reserve A for non empty set;
reserve i,j,k for Element of NAT;
reserve a,b,c,d for Real;
reserve y,r,s,x,t,w for Element of RAT+;

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;
