One more cool thing is that you can express different levels of
abstraction in the same DSL. It falls out naturally once you have
specified enough combinators that the higher-level ones output the
lower-level ones.

For example, consider a directory with the following files:

srfi/1.sld
srfi/1/lists.sld
srfi/13.sld
srfi/13/strings.sld

The high-level combinators (using-convention ".sld" (directory ".")),
when applied to the above directory tree, would expand into the
following low-level combinators:

(at srfi
     (union (at 1
                (union (library "srfi/1.sld")
                       (at lists (library "srfi/1/lists.sld"))))
            (at 13
                (union (library "srfi/13.sld")
                       (at strings (library "srfi/13/strings.sld"))))))

This way, the REPL could offer a "macroexpand"-style tool to show what
the library finder is actually doing. You could also write the
expansions into build logs and things like that.