What passes in the inputs to the lambda?

[jgart]

In the following snippet, what passes in the inputs to the lambda?

(modify-phases %standard-phases
  (replace 'unpack
    (lambda* (#:key inputs #:allow-other-keys)
      (let* ((source (assoc-ref inputs "source"))
             (guile-dir (assoc-ref inputs "guile"))
             (guile (string-append guile-dir "/bin/guile")))
        (invoke guile "--no-auto-compile" source)
        (chdir "bootar"))))