  Transformation method provides an efficient way to control wave propagation
by materials. However, the degree to which this transformation concept can be
applied to other physical phenomena remains an open question. Recently, Hu et
al. presented a general framework for determining the transformation relations
of the physical quantities in arbitrary partial differential equation (PDE) in
its application scope according to the idea of transformation method. In this
paper, we will review the main concepts of this general theory, particularly
the difference between this idea and usual methods. The flexibility of this
method is shown using an example.
