The expander recognizes top-level begin expressions, and interleaves the evaluation and expansion of of the begin body, so that compile-time expressions within the begin body affect later expansions within the body. (In other words, it ensures that expanding a begin is the same as expanding separate top-level expressions.)
The stx should have a context already, possibly introduced with namespace-syntax-introduce.
|→ (listof syntax?)|
|stx : syntax?|