I got two little questions concerning stacks in `expl3`.
1. What is the best way to build a stack for a sequence?
The naive way would be to use something like
\seq_push:No \l_my_stack_seq \l_my_seq
to push and then
\seq_pop:NN \l_my_stack_seq \l_my_seq
But this doesn't look like a clean usage of `\seq_pop:NN` and
2. Should there be a higher level interface for stacks of the different
data types in `expl3`?