[R] Taking code from packages

[Ales Ziberna]

Hello!

I am currently in the process of creating (my first) package, which (when
ready) I intend to publish to CRAN. In the process of creating this package
I have taken some code form existing packages. I have actually copied parts
of functions in to new functions. This code is usually something very basic
such as Rand index. What is the proper procedure for this?

Since most of R (and also the packages I have taken code form) is published
under GPL, I think this should be OK. However I do not know if:
1.	I should still ask authors of the packages for permission or at
least notify them.
2.	Ad references to the functions (and packages) from which I had taken
the code or only to the references they use.

What about regarding code that was sent to the list, usually as a response
to one of my problems. I assume that in this case it is best to consult the
author?

Any comments and opinions are very welcomed!

Best regards,
Ales Ziberna