2014-05-24  Youyi Fong
    * create package

2014-7-7
    * change the algorithm to use sdc.3.deriv.r in step 2.
    * version shared with Yunda, Allan and Peter

2014-10-7
    * make changes so that x and y and r are understood as to be on the log scale

2014-11-4
    * manuscript submitted
    * changed name from sdc to prc
