Git branch dependency between stan and lib/stan_math

how do I get the dependencies right between a branch of stan and lib/stan_math?

this PR is failing:

I started this branch in order to add vector and matrix expressions - this required adding a specialization to lib/stan_math that was merged into the develop branch.
the PR is testing against the old version of lib/stan_math, which fails.
what is the proper incantation to make it run against the current develop branch of lib/stan_math?

is this in the wiki? couldn’t find it.

problem solved - updates to stan/math submodule generate a PR for stan repo - merged and all is good.