R devel - Nov 2014 - Appropriately Crediting Use of R Source Code in Package

I'm developing packages that in some places use code that started off as a
direct copy-paste from the R sources (this is C code).  The finished code is
fairly different from what I started with, but clearly uses similar algorithms /
tricks as the original code, and some lines here and there are still verbatim
copies.

What is the correct way to credit the R developers in DESCRIPTION? As copyright
holders?  As contributors / authors (though it almost seems presumptuous for me
to associate them with my crappy package)?  If so, should I use "R Core
Team" as seems to be the standard in base R packages for authors?

I've looked in Writing R extensions, the 6/2012 R Journal article, and other
places and can't quite find an answer for my use case (I may be obtuse).
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