I have a couple of general questions about expl3.

- Are all boxes and coffins made so that their content is wrapped in a TeX group?
  More specifically, is it safe for code to assume that all box/coffin content is
  implicity grouped?

- Many functions in expl3 are of the form `a := func(b, c)` (e.g. `\str_concat:NNN`,
  `\seq_set_map:NNn`) or `a := func(b)` (e.g. `\int_set:Nn`, `\tl_set:Nn`).
  Sometimes, one will want to write code like `a := func(a, b)` or `a := func(a)`,
  that is, directly assign a new value to a variable based on its current value.
  Is it ever safe to write such code in expl3?