code: public Matrix4 setToReflection (float x, float y, float z, float w) {
          float x2 = -2f*x, y2 = -2f*y, z2 = -2f*z;
          float xy2 = x2*y, xz2 = x2*z, yz2 = y2*z;
          float x2y2z2 = x*x + y*y + z*z;
          return set(1f + x2*x, xy2, xz2, x2*w*x2y2z2,
                     xy2, 1f + y2*y, yz2, y2*w*x2y2z2,
                     xz2, yz2, 1f + z2*z, z2*w*x2y2z2,
                     0f, 0f, 0f, 1f);
      }
docs: Sets this to a reflection across the specified plane.
  
  @return a reference to this matrix, for chaining.