Sat Aug 14  2004  D tsukahara
	* derivative 0.1 make
	  * Recast document in English
	  * add test script end of derivative.rb
Fri Aug 13  2004  D tsukahara && T horinouti
	* derivative 0.0.4 make
	  * add 2nd order derivative method (given by T horinouti)
Fri Aug 6  2004  D tsukahara
	* derivative 0.0.1 make (have not been released)
