This is from NumFOCUS’s lawyer
"Who owns the copyright is dictated by copyright law as that may then be altered by any private agreements (such as X’s agreement with Y that he owns the copyright, not Y). From the description, it would appear that the copyright owners are some combination of Columbia University, X, and possibly Y (if there were other Y developers besides X who worked on it).
As to what to say in the file, they should just add another line to the file header(s) that is "© [year of publication] [correct assortment of copyright owners, e.g., “Columbia University and X]. They don’t need to specify (i) what they are the copyright owners of in the file or (ii) what their individual copyright ownership interest is, whether it might be that each owns different parts of the new functionality or they own the whole thing jointly. It sounds most likely to be the latter, but it’s not something that has to be figured out for purposes of including the copyright notice to the file. Because it sounds like joint ownership I would use the construct I mention above, “[entity] and [entity],” rather than two separate notices, but they could also do “© [year of publication] Columbia University” on one line and “© [year of publication] X” on a second line and that would also be acceptable.”
The original question was:
"Let me briefly clarify. This question is coming from a so-called pull-request for which Columbia employees and myself have worked on. Thus, a number of C++ files have been edited/created by Columbia staff and myself. While our source control system would likely be able to tell who edited what line of code, it can definitely not be separated into functionally meaningful modules.
So it’s basically that Columbia staff and myself did write a new feature for Stan. The question now is what we should put as copyright for the files and functions created? Would it be possible that the copyright for the contribution is held by Columbia and myself or would that complicate things? What would be the recommended way to handle this?
BTW, code I contribute to Stan is granted by Y (X’s employer) to me such that I hold the copyright.