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diff --git a/src/l-system.lisp b/src/l-system.lisp
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--- /dev/null
+++ b/src/l-system.lisp
@@ -0,0 +1,56 @@
+;;;; l-system.lisp
+#|
+(ql:quickload :l-system)
+|#
+
+(in-package #:l-system)
+
+;;; "l-system" goes here. Hacks and glory await!
+
+(defun iterconcat (fn list)
+  "Applies fn on each element of list, and concatenate a copy of the resulting lists."
+  (iter (for item in list)
+	(appending (funcall fn item))))
+
+(defun make-case-clause (keys value)
+  "Make one CASE clause mapping a list of KEYS to one VALUE"
+  `((,@keys) ',value))
+
+(defun make-case-clauses-from-rules (rules)
+  "Makes a list of CASE clauses from the RULES."
+  (mapcar (lambda (rule) (make-case-clause (list (first rule)) (rest rule)))
+          rules))
+
+(defun generate-l-system (rules)
+  "Make lambda Lindenmayer system"
+  `(lambda (atom) (case atom ,@(make-case-clauses-from-rules rules))))
+
+(defmacro l-system (&rest rules)
+  (generate-l-system rules))
+
+(defun iter-l-system (fn seed n)
+  (iter (repeat n)
+	(with item = seed)
+	(setf item
+	      (iterconcat fn
+			  item))
+	(finally (return item))))
+
+#|
+(iterconcat (l-system (a a i a d a)
+		      (b b d b i b)
+		      (i i b i a i)
+		      (d d a d b d))
+	    '(d)) ;; --> (D A D B D)
+
+(iter-l-system (l-system (a a i a d a)
+			 (b b d b i b)
+			 (i i b i a i)
+			 (d d a d b d))
+	       '(d)
+	       3)
+;; --> (D A D B D A I A D A D A D B D B D B I B D A D B D A I A D A I B I A I A I A D
+        A D A D B D A I A D A D A D B D A I A D A D A D B D B D B I B D A D B D B D B
+        I B D A D B D B D B I B I B I A I B D B I B D A D B D A I A D A D A D B D B D
+        B I B D A D B D)
+|#
diff --git a/src/package.lisp b/src/package.lisp
new file mode 100644
index 0000000..db6f96e
--- /dev/null
+++ b/src/package.lisp
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+;;;; package.lisp
+
+(defpackage #:l-system
+  (:use #:cl #:iter)
+  (:export #:l-system
+	   #:turtle))
+