diff --git a/example/fancy.build b/example/fancy.build
index 60f35b7..b79853d 100644
--- a/example/fancy.build
+++ b/example/fancy.build
@@ -1,62 +1,11 @@
-(in-package :fancybuild)
-
 (defun runp (cmd)
   (assert (zerop (uiop:wait-process (uiop:launch-program cmd :output *standard-output* :error-output *error-output*)))))
 
-(defun target (name target)
-  (setf (gethash name *targets*) target))
-
-(defun mtime (x) (osicat-posix:stat-mtime (osicat-posix:stat x)))
-; todo blake3?
-(defun sha256 (fn)
-  (with-open-file (fp fn
-                      :direction :input
-                      :if-does-not-exist :error
-                      :element-type '(unsigned-byte 8))
-    (let ((buf (make-array (file-length fp) :element-type '(unsigned-byte 8))))
-      (read-sequence buf fp)
-      (ironclad:digest-sequence :sha256 buf))))
-
-(defun any (pred &rest lis)
-  (if (null (car lis))
-    nil
-    (or (apply pred (mapcar #'car lis))
-        (apply #'any (cons pred (mapcar #'cdr lis))))))
-(defun all (pred &rest lis)
-  (if (null (car lis))
-    t
-    (and (apply pred (mapcar #'car lis))
-         (apply #'all (cons pred (mapcar #'cdr lis))))))
-
-(defun freeze-file-state (fpath)
-  (cons (mtime fpath) (sha256 fpath)))
-
-(defun has-file-changed (fpath state)
-  (and (not (= (mtime fpath) (car state)))
-       (not (equalp (sha256 fpath) (cdr state)))))
-
-; Rebuild whenever any of the output files doesn't exist
-; Or, whenever any of the input files has a newer mtime than any of the output files
-;  (that is, whenever the newest input file is newer than the oldest output file)
-; Or, whenever any of the input files has been modified
-(defclass rebuilt-on-change (buildable)
-  ((input-files :initarg :input-files :initform nil)
-   (output-files :initarg :output-files :initform nil)))
-
-(defmethod snapshot ((b rebuilt-on-change))
-  (mapcar #'freeze-file-state (slot-value b 'input-files)))
-
-(defmethod is-dirty ((b rebuilt-on-change) old)
-  (or (not (any #'probe-file (slot-value b 'output-files)))
-      (> (reduce #'max (mapcar #'mtime (slot-value b 'input-files)))
-	 (reduce #'min (mapcar #'mtime (slot-value b 'output-files))))
-      (any #'has-file-changed (slot-value b 'input-files) old)))
-
-(defclass c-source-buildable (rebuilt-on-change)
+(defclass c-source-buildable (rebuilt-on-file-change)
   ((source-file :initarg :source-file)
    (obj-file :initarg :obj-file)))
 
-(defclass c-binary-buildable (rebuilt-on-change)
+(defclass c-binary-buildable (rebuilt-on-file-change)
   ((target-binary :initarg :target-binary)
    (obj-files)))
 
diff --git a/fb.asd b/fb.asd
index 8e7ccae..92b49cb 100644
--- a/fb.asd
+++ b/fb.asd
@@ -9,6 +9,7 @@
     :depends-on (#:osicat #:ironclad #:cl-conspack)
 
     :components ((:file "dependable")
+		 (:file "buildables/rebuilt-on-file-change" :depends-on ("dependable"))
 		 (:file "fb-entry" :depends-on ("dependable")))
 		 ;(:file "x" :depends-on ("y" "z"))
 
diff --git a/src/buildables/rebuilt-on-file-change.lisp b/src/buildables/rebuilt-on-file-change.lisp
new file mode 100644
index 0000000..be148ba
--- /dev/null
+++ b/src/buildables/rebuilt-on-file-change.lisp
@@ -0,0 +1,46 @@
+(in-package :fancybuild)
+
+(export '(rebuilt-on-file-change input-files output-files))
+
+(defun mtime (x) (osicat-posix:stat-mtime (osicat-posix:stat x)))
+; todo blake3?
+(defun sha256 (fn)
+  (with-open-file (fp fn
+                      :direction :input
+                      :if-does-not-exist :error
+                      :element-type '(unsigned-byte 8))
+    (let ((buf (make-array (file-length fp) :element-type '(unsigned-byte 8))))
+      (read-sequence buf fp)
+      (ironclad:digest-sequence :sha256 buf))))
+
+(defun any (pred &rest lis)
+  (if (null (car lis))
+    nil
+    (or (apply pred (mapcar #'car lis))
+        (apply #'any (cons pred (mapcar #'cdr lis))))))
+(defun all (pred &rest lis)
+  (if (null (car lis))
+    t
+    (and (apply pred (mapcar #'car lis))
+         (apply #'all (cons pred (mapcar #'cdr lis))))))
+
+(defun freeze-file-state (fpath)
+  (cons (mtime fpath) (sha256 fpath)))
+
+(defun has-file-changed (fpath state)
+  (and (not (= (mtime fpath) (car state)))
+       (not (equalp (sha256 fpath) (cdr state)))))
+
+; Rebuild whenever any of the output files doesn't exist
+; Or, whenever any of the input files has been modified
+(defclass rebuilt-on-file-change (buildable)
+  ((input-files :initarg :input-files :initform nil)
+   (output-files :initarg :output-files :initform nil)))
+
+(defmethod snapshot ((b rebuilt-on-file-change))
+  (mapcar #'freeze-file-state (slot-value b 'input-files)))
+
+(defmethod is-dirty ((b rebuilt-on-file-change) old)
+  (or (not (any #'probe-file (slot-value b 'output-files)))
+      (any #'has-file-changed (slot-value b 'input-files) old)))
+
diff --git a/src/dependable.lisp b/src/dependable.lisp
index 6b217c4..82605a2 100644
--- a/src/dependable.lisp
+++ b/src/dependable.lisp
@@ -1,6 +1,10 @@
 (defpackage #:fancybuild
   (:use #:cl)
-  (:export #:main))
+  (:export #:buildable #:dependencies #:cached-snapshot #:pretty-name #:unique-key #:snapshot #:do-build #:is-dirty))
+
+; package in which build scripts run
+(defpackage #:fancily-built
+  (:use #:cl #:fancybuild))
 
 (in-package :fancybuild)
 
diff --git a/src/fb-entry.lisp b/src/fb-entry.lisp
index 2fd38bf..00789b7 100644
--- a/src/fb-entry.lisp
+++ b/src/fb-entry.lisp
@@ -1,8 +1,13 @@
 (in-package :fancybuild)
 
+(export '(target main))
+
 (defparameter *targets* (make-hash-table :test #'equal))
 (defparameter *global-cache* (make-hash-table :test #'equal))
 
+(defun target (name target)
+  (setf (gethash name *targets*) target))
+
 (defun naively-build (b)
   (mapcar #'naively-build (slot-value b 'dependencies))
   (when (and (not (slot-value b 'cached-snapshot))
@@ -38,7 +43,9 @@
         (setf *global-cache*
               (conspack:with-interning () (conspack:decode buf))))))
 
-  (load "fancy.build")
+  (progn
+    (in-package :fancily-built)
+    (load "fancy.build"))
 
   (mapcar #'(lambda (target-name)
               (let ((target (gethash target-name *targets*)))