For GC purposes, if a "prefix" (a closure frame that caprues
top-level or module-level bindings) may refer to syntax objects
that are not used by any reachable closure, in which case the
syntax object can be dropped. This pruning of syntax objects
uses the infrastructure already in place to prune variables.
Syntax objects were not included in the original pruning
implementation, because they are unlikely to create
finalization cycles in the way that global-variable
references can. A syntax object can retain a namespace's
table of module imports, however, which can be substantial
and worth releasing of a closure is only held, say, for
a low-level finalization action.
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