reserve a, b for Real;
reserve RNS for RealNormSpace;
reserve x, y, z, g, g1, g2 for Point of RNS;

theorem Th7:
  ||.x - y.|| = ||.y - x.||
proof
  x - y = - (y - x) by RLVECT_1:33;
  hence thesis by Th2;
end;
