pip requirements.txt with alternative index

I want to put all the requirements of a repoze Zope2 install in a pip requirements file. Most of the repoze packages don't seem to be on PyPi, but there's an alternative PyPi index for them here. But I can't figure out how to tell pip to use that index together with a requirements file. For single packages, it's easy

pip install zopelib -i http://dist.repoze.org/zope2/2.10/simple/

I tried the following

pip install -r requirements.txt -i http://dist.repoze.org/zope2/2.10/simple/

or in my requirements.txt all kind or permutations of these:

zopelib -i http://dist.repoze.org/zope2/2.10/simple/
zopelib --index http://dist.repoze.org/zope2/2.10/simple/
-i http://dist.repoze.org/zope2/2.10/simple/ zopelib

or (because the documentation says "Note that all these options must be on a line of their own.")

--index http://dist.repoze.org/zope2/2.10/simple/
zopelib

So, what's the correct way of telling pip to use http://dist.repoze.org/zope2/2.10/simple/ as index?

requirements.txt:

-i http://dist.repoze.org/zope2/2.10/simple
zopelib

Example:

$ pip install -r requirements.txt
...
Successfully installed zopelib

Add an extra index location to the requirements file just before the package/project name:

--extra-index-url <Extra URLs other than index-url>
<some_project_name>

Alternatively, you may use -i or --index-url <Base URL of the Python Package Index>.

Refer: requirements file format