2.5 Variable References and #%top
Refers to a module-level or local binding, when id
not bound as a transformer (see Expansion (Parsing)
). At run-time,
the reference evaluates to the value in the location
When the expander encounters an id that is not bound by a
module-level or local binding, it converts the expression to
(#%top . id) giving #%top
the lexical context of the id; typically, that context refers
to #%top. See also Expansion Steps.
Refers to a module-level or top-level definition that could bind
id, even if id has a local binding in its context.
Within a module form, (#%top . id) expands to just
id—with the obligation that id is defined within
the module. At phase level 0, (#%top . id) is an
immediate syntax error if id is not bound. At phase
level 1 and higher, a syntax error is reported if id is not
defined at the corresponding phase by the end of module-body
See also Expansion Steps for information on how the expander
introduces #%top identifiers.