All expressions here are reachable
(having been derived from named statements).
This expression is on its own unreachable,
intended to be expanded into another expression.
Both reachable and unreachable,
with the intent of expanding into an expression but also providing
itself with supporting expressions.
The above expression will end at depth N+1,
to be auto-closed.
The below expression will yield an Ident->Expr,
and so will _begin_ at N+1.
We must therefore ensure,
and this test do so assert,
that this matching depth does not cause the reparenting of this next
expression into its preceding sibling.