Fixtype of static assertion declarations [C:6.7.10] [C:A.2.2].
This is a product type introduced by fty::defprod.
This corresponds to static_assert-declaration in the grammar in [C].
We use a list of string literals, which should be non-empty, but we do not capture this constraint. This mirrors the ABNF grammar: this way, we preserve the information about adjacent string literals.