=begin

=Reference manuals of the Derivative library

* ((<module NumRu::Derivative|URL:numru-derivative.html>)) :
  Module functions of Derivative Operater for NArray.

* ((<module NumRu::GPhys::Derivative|URL:gphys-derivative.html>)) :
  Module functions of Derivative Operater for GPhys.

* ((<description of mathmatical definition|URL:math-doc/document/index.html>))
  Description of mathmatical definition about Derivative.

=end