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can gpl code contain more restrictively licensed code? (CC BY-NC-ND 3.0)

There is a wordpress theme I am modifying to create my own derived work. This original wordpress theme has the GPLv2 licence in it's root directory, so I assume that the entire theme is GPL code.

There is a particular php file which has the following written in its first few lines:

// This code is protected under Creative Commons License: http://creativecommons.org/licenses/by-nc-nd/3.0/

Is that legal for them to make a part of the code under a more restrictive license? The CC license referenced is more restrictive because it prohibits derivative work, but that is exactly what I am doing is making a derivative work.

So here are my two questions:

  • By having the full GPL (version 2) licence in the root of the theme's directory, is that a legally binding way of saying that the code is licensed under the GPL2?
  • Is that legal for them to make that file licensed under that particular CC license?

Ultimately, what I care about is this: I want to make sure that what I am doing is technically legal :)

None of the source files actually say that it is under GPL; except the theme does include jQuery and jQuery plugins (under MIT and GPL2) as well as thumb.php which explicitly says it is under GPL2.