This is why I raised an eyebrow when TFA wrote,

> When I saw this, I wondered why it has several inner hashes instead of using the raw string.

The inner hash constrains the alphabet on that portion of the input to the outer hash, thus easily letting you use a separator like "," or "|" without having to deal with the alphabet of the inner input, since it gets run through a hash. That is, for a very simplistic use case of two inputs a & b:

  sha256(','.join(
    [sha256(a), sha256(b)]
  ))

If one is familiar with a git tree or commit object, this shouldn't be unfamiliar.

Now … whether that's why there was an inner hash at that point in TFA's code is another question, but I don't think one should dismiss inner hashes altogether.