[R] want to write a package for R

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     hello,everyone!
     I have studied in univ. for two years.and my teacher have put out 
some stat. model .and i want to write it to a R package, and take 
it to my future paper.any advise? i learned something about SAS/stat.
and i have read the tutorial of R.
     now i can interactive with R,and
write R file.but i didn't have a contour of how to write a package.
R extension said package is consisted of DESCRIPTION,INDEX,subdirectories
,bundles.should i write all the file?
     thank you.