code: public Matrix3 setToReflection (float x, float y, float z) {
          float x2 = -2f*x, y2 = -2f*y, z2 = -2f*z;
          float xy2 = x2*y, xz2 = x2*z, yz2 = y2*z;
          return set(1f + x2*x, xy2, xz2,
                     xy2, 1f + y2*y, yz2,
                     xz2, yz2, 1f + z2*z);
      }
docs: Sets this to a reflection across a plane intersecting the origin with the supplied normal.
  
  @return a reference to this matrix, for chaining.