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> * <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;
