Which is why this kata should not be translated to some languages.

While before (lambda here) is perhaps sensibly disallowed, it actually does have a sensible interpretation too. If you think of de Bruijn indices as providing a "stack" of arguments in context then before pops the stack before continuing into a subexpression. So, for instance

lambda (before (lambda here))   ~   flip const

In other terms, before will "shield" a subexpression from one level of context.