[racket] Quick question on units, composition

Hi friends,

Just a quick question on using Units -- I have one unit that imports a
signature and appends a prefix, a la

    (define-unit my-unit@
       (import (prefix config: config-signature^))
       (export my-unit^)
       ;; implements my-unit^...
       )

But when I attempt to invoke this after invoking a unit satisfying the
dependency signature (above, `config-signature`), I get errors that it
can't locate identifiers from the imported signatures.

I suspect this is to do with the fact that I'm invoking the dependency at
the toplevel using define-values/invoke-unit/infer, per the Racket Guide,
but a perusal of the Racket Reference suggests another method of invocation
might be more suitable, since define-values is just exporting bindings to
the global namespace, and here I'm referring to them by a prefixed form.

That said, I'm not entirely clear on what what value the `invoke-units`
syntax returns, or what side effects invocation of a unit even *does* from
reading the docs<http://docs.racket-lang.org/reference/invokingunits.html#%28form._%28%28lib._racket%2Funit..rkt%29._invoke-unit%29%29>(if
it isn't defining bindings).

Any help would be appreciated! Thanks ^_^

-Paul
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