Replace indexing block model structure with linear algebra?

[ctross]

We currently let users have block models based on known group identities, using a syntax like:

B[group[i], group[j]]

Better would be to give i and j simplices, W[i] and W[j], which can be binary indicators for known data and parameter vectors if missing. Then we have:

W[i] B T(W[j])

in place of B[group[i], group[j]].

Credit: This was suggested by an anonymous reviewer at Royal Society Open Science.