theorem Th3:
  [x1,x2,x3] = [y1,y2,y3] implies x1 = y1 & x2 = y2 & x3 = y3
proof
  assume
A1: [x1,x2,x3] = [y1,y2,y3];
  then [x1,x2] = [y1,y2] by Th1;
  hence thesis by A1,Th1;
end;
