2.6 Locations: #%variable-reference
(#%variable-reference id) (#%variable-reference (#%top . id)) (#%variable-reference)
Produces an opaque variable reference value representing the
location of id, which must be bound as a variable. If
no id is supplied, the resulting value refers to an
“anonymous” variable defined within the enclosing context (i.e.,
within the enclosing module, or at the top level if the form is not
inside a module).
A variable reference can be used with variable-reference->empty-namespace, variable-reference->resolved-module-path, and variable-reference->namespace, but facilities like define-namespace-anchor and namespace-anchor->namespace wrap those to provide a clearer interface. A variable reference is also useful to low-level extensions; see Inside: Racket C API.