I know it is not equivalent—that is why I proposed it as a sensible alternative to what the PEP proposes (which I quoted). The punctuation may have made it unclear, but what I was trying to say was:

> … a sensible alternative to [PEP proposal]; namely, just contract on the inner indices.

and not

> … a sensible alternative to contracting on the inner indices.

My argument for why it's sensible is precisely what you mentioned, namely, that it is what a mathematician would expect.