Today I spent some research time pair-coding (yet more) with Price-Whelan the fully marginalized likelihood ratio test we are using for our comoving-stars project. There are subtleties! Every time we look at the code we find bugs and think-os. I am reminded of the point that we generally find bugs continuously and with no sign of slowing until we are happy that the code seems to pass our unit and functional tests, at which point we stop looking. That doesn't mean the code is correct! A corollary: Almost no-one has ever wasted time testing code. It turns out that one of the most troublesome parts of this project is units conversion. Not surprising, really, when we have AU, pc, km/s, mas/yr, the inverse squared of these (in inverse variance matrices) and many, many more.