(defun all-dependencies (system)
  (let ((ht (make-hash-table :test #'equal)))
    (labels ((recur (system)
               (unless (gethash system ht)
                 (let ((deps (ql:who-depends-on system)))
                   (setf (gethash system ht) t)
                   (dolist (dep deps)
                     (recur dep))))))
      (recur system)
      (alexandria:hash-table-keys ht))))

CL-USER> (all-dependencies "hu.dwim.asdf")
("hu.dwim.zlib" "varint-test" "swank-crew-test" "swank-client-test"
 "snappy-test" "sip-hash-test" "scribble/test" "screamer-tests"
 "reader-interception-test" "random-test" "pileup-tests"
 "parser-combinators-tests" "package-renaming-test" "inferior-shell/test"
 "hu.dwim.zlib/test" "hu.dwim.stefil/test" "incf-cl-test"
 "hu.dwim.stefil+swank" "firephp-tests" "fare-utils-test"
 "fare-quasiquote-test" "fare-memoization/test" "com.google.flag-test"
 "com.google.base-test" "cl-m4-test" "cl-l10n/test" "cl-custom-hash-table-test"
 "cl-arrows-test" "city-hash-test" "cffi-objects.tests" "bencode-test"
 "babel-tests" "asdf-finalizers-test/2" "asdf-finalizers-test/1"
 "asdf-finalizers-test" "asdf-encodings/test" "arrows/test" "acm-random-test"
 "hu.dwim.stefil" "hu.dwim.sdl/ttf" "hu.dwim.sdl/image" "hu.dwim.sdl/gfx"
 "hu.dwim.sdl" "cl-graph+hu.dwim.graphviz" "hu.dwim.graphviz"
 "hu.dwim.defclass-star+swank" "hu.dwim.defclass-star+contextl"
 "hu.dwim.defclass-star" "hu.dwim.stefil+hu.dwim.def"
 "hu.dwim.defclass-star+hu.dwim.def+contextl"
 "hu.dwim.defclass-star+hu.dwim.def" "hu.dwim.stefil+hu.dwim.def+swank"
 "hu.dwim.def+swank" "hu.dwim.def+contextl" "hu.dwim.def+cl-l10n" "hu.dwim.def"
 "hu.dwim.common-lisp" "hu.dwim.syntax-sugar/unicode" "hu.dwim.syntax-sugar"
 "hu.dwim.graphviz.test" "hu.dwim.defclass-star.test" "hu.dwim.def.test"
 "hu.dwim.zlib/fancy" "hu.dwim.walker.test"
 "hu.dwim.syntax-sugar/lambda-with-bang-args" "hu.dwim.syntax-sugar.test"
 "trace-db" "software-evolution-library/utility"
 "software-evolution-library/stefil-plus"
 "software-evolution-library/serapi-io" "software-evolution-library/lisp-diff"
 "software-evolution-library/ast-diff-html"
 "software-evolution-library/ast-diff" "software-evolution-library/view"
 "software-evolution-library/test" "software-evolution-library/mongo"
 "software-evolution-library/clang-instrument"
 "software-evolution-library/clang-diff" "software-evolution-library"
 "repair-clang" "repair" "oclcl-test" "neutral" "gamebox-math.vari"
 "gamebox-math.tests" "gamebox-math" "umbra" "shadow" "game-math"
 "golden-utils" "cl-bson-test" "cl-bson" "arrow-macros-test" "arrow-macros"
 "hu.dwim.walker" "hu.dwim.util/source" "hu.dwim.util/mop"
 "hu.dwim.util/linear-mapping" "hu.dwim.util/i18n"
 "hu.dwim.util/error-handling+swank" "hu.dwim.util/error-handling"
 "hu.dwim.util/command-line" "hu.dwim.util/temporary-files"
 "hu.dwim.util+iolib" "hu.dwim.uri.test" "hu.dwim.uri"
 "hu.dwim.serializer.test" "cl-mw.examples.with-task-policy"
 "cl-mw.examples.ping" "cl-mw.examples.monte-carlo-pi"
 "cl-mw.examples.higher-order" "cl-mw.examples.hello-world"
 "cl-mw.examples.argument-processing" "cl-mw" "hu.dwim.serializer"
 "hu.dwim.reiterate" "hu.dwim.quasi-quote.xml+cxml" "hu.dwim.quasi-quote.xml"
 "hu.dwim.quasi-quote.pdf" "hu.dwim.quasi-quote.xml+hu.dwim.quasi-quote.js"
 "hu.dwim.quasi-quote.js" "hu.dwim.quasi-quote.test" "hu.dwim.quasi-quote.css"
 "hu.dwim.quasi-quote" "hu.dwim.util/threads" "hu.dwim.util/flexml"
 "hu.dwim.util/finite-state-machine" "hu.dwim.def.namespace" "hu.dwim.util"
 "hu.dwim.sdl/fancy" "projectured.swank" "projectured.test"
 "projectured.sdl.test" "projectured.executable" "projectured.sdl"
 "projectured.projection" "projectured.document" "projectured.editor"
 "libssh2.test" "libssh2" "hu.dwim.util/worker-group" "hu.dwim.util/soap"
 "hu.dwim.reiterate+hu.dwim.logger" "hu.dwim.rdbms.test"
 "hu.dwim.rdbms.sqlite.test" "hu.dwim.rdbms.sqlite"
 "hu.dwim.rdbms.postgresql.test" "hu.dwim.rdbms.postgresql"
 "hu.dwim.rdbms.all.test" "hu.dwim.rdbms.oracle.test" "hu.dwim.rdbms.all"
 "hu.dwim.rdbms.oracle" "hu.dwim.rdbms" "hu.dwim.util/authorization"
 "hu.dwim.partial-eval.test" "hu.dwim.partial-eval" "hu.dwim.logger.test"
 "hu.dwim.logger+swank" "hu.dwim.logger+iolib" "hu.dwim.logger"
 "hu.dwim.util/standard-process" "hu.dwim.delico.test"
 "hu.dwim.def+hu.dwim.delico" "hu.dwim.delico" "hu.dwim.web-server.websocket"
 "hu.dwim.walker.documentation" "hu.dwim.util.documentation"
 "hu.dwim.syntax-sugar.documentation" "hu.dwim.serializer.documentation"
 "hu.dwim.rdbms.documentation" "hu.dwim.quasi-quote.documentation"
 "hu.dwim.presentation+hu.dwim.stefil"
 "hu.dwim.presentation+cl-graph+cl-typesetting"
 "hu.dwim.presentation+cl-typesetting" "hu.dwim.partial-eval.documentation"
 "hu.dwim.logger.documentation" "hu.dwim.graphviz.documentation"
 "hu.dwim.delico.documentation" "hu.dwim.defclass-star.documentation"
 "hu.dwim.def.documentation" "hu.dwim.common-lisp.documentation"
 "hu.dwim.presentation" "hu.dwim.web-server.application"
 "hu.dwim.web-server+swank" "hu.dwim.presentation+hu.dwim.web-server"
 "hu.dwim.web-server" "weblocks-perec"
 "hu.dwim.web-server.application+hu.dwim.perec" "hu.dwim.perec.sqlite"
 "hu.dwim.util/production" "hu.dwim.util.test" "hu.dwim.perec.postgresql"
 "hu.dwim.perec.all" "hu.dwim.perec.oracle" "hu.dwim.perec+swank"
 "hu.dwim.perec+iolib" "hu.dwim.perec.sqlite.test"
 "hu.dwim.perec.postgresql.test" "hu.dwim.perec.documentation"
 "hu.dwim.perec.all.test" "hu.dwim.perec.oracle.test" "hu.dwim.perec.test"
 "hu.dwim.perec+hu.dwim.quasi-quote.xml" "hu.dwim.perec"
 "hu.dwim.computed-class+swank" "hu.dwim.web-server.documentation"
 "hu.dwim.web-server.application.test" "hu.dwim.web-server.test"
 "hu.dwim.computed-class.documentation" "hu.dwim.computed-class.test"
 "hu.dwim.computed-class+hu.dwim.logger" "hu.dwim.computed-class"
 "hu.dwim.def+hu.dwim.common" "hu.dwim.reiterate/test"
 "hu.dwim.debug.documentation" "hu.dwim.debug.test" "hu.dwim.debug"
 "hu.dwim.common.documentation" "hu.dwim.common" "hu.dwim.bluez/fancy"
 "hu.dwim.bluez" "hu.dwim.asdf.documentation" "hu.dwim.asdf")
CL-USER> (length *)
223