diff --git a/util.lisp b/util.lisp
index a363df0..c645166 100644
--- a/util.lisp
+++ b/util.lisp
@@ -202,7 +202,11 @@
 	"Return n plus a random offset"
 	(+ n (- (random (* 2 max-offset)) max-offset)))
 
-(defun chancep (percent)
+(defun chancep (p)
+	"Do a random test with 1/p probability of success."
+	(= 1 (random p)))
+
+(defun probabilityp (percent)
 	"Do a random test, with the percentage giving the success probability"
 	;;Accuracy: 0.1
 	(> percent (/ (random 1000) 10)))

diff --git a/util.lisp b/util.lisp
index a363df0..c645166 100644
--- a/util.lisp
+++ b/util.lisp
@@ -202,7 +202,11 @@
 	"Return n plus a random offset"
 	(+ n (- (random (* 2 max-offset)) max-offset)))
 
-(defun chancep (percent)
+(defun chancep (p)
+	"Do a random test with 1/p probability of success."
+	(= 1 (random p)))
+
+(defun probabilityp (percent)
 	"Do a random test, with the percentage giving the success probability"
 	;;Accuracy: 0.1
 	(> percent (/ (random 1000) 10)))
diff --git a/world.lisp b/world.lisp
index 40bcffb..5f79f46 100644
--- a/world.lisp
+++ b/world.lisp
@@ -6,7 +6,7 @@
 ;;;; (c) 2018 Daniel Vedder, MIT license
 ;;;;
 
-(defvar *world* NIL)
+(defparameter *world* NIL)
 (defconstant *directions* '(N NE E SE S SW W NW))
 
 (defstruct patch
@@ -100,17 +100,21 @@
 
 ;; TOPOGRAPHY FUNCTIONS
 
+(defun print-topography (&optional (stream T) (world *world*))
+	"Print a text representation of the world and each patch's biome"
+	(dolist (row world)
+		(format stream "~&~A~%"
+			(string-from-list
+				(mapcar #'(lambda (p) (biome-char (patch-biome p))) row)
+				""))))
+
 (defun save-topography (file-name &optional (world *world*))
 	"Save the world topography as a text file"
 	(debugging "~&Saving world to file ~A" file-name) ;debug
 	(with-open-file (tf file-name :direction :output)
-		(dolist (row world)
-			(format (if file-name tf T) "~&~A~%"
-				(string-from-list
-					(mapcar #'(lambda (p) (biome-char (patch-biome p))) row)
-					" ")))))
-	
-(defun generate-biomes (size-factor &optional (world *world*))
+		(print-topography tf)))
+
+(defun generate-biomes (size-factor world)
 	;;XXX The maps this produces don't look as expected, but for
 	;; current purposes they are good enough
 	(debugging "~&Generating biomes") ;debug
@@ -134,18 +138,18 @@
 					(setf (patch-biome (coord x y world))
 						(get-biome b)))))))
 	
-(defun generate-stream (x0 y0 &optional (world *world*))
+(defun generate-stream (x0 y0 world)
 	(debugging "~&Generating a stream, starting at ~S/~S" x0 y0) ;debug
 	(do* ((dir (random-elt *directions*)
-			  (if (chancep 60) dir (next-dir dir (random-elt '(T NIL)))))
+			  (if (probabilityp 60) dir (next-dir dir (random-elt '(T NIL)))))
 			 (patch (coord x0 y0 world) (neighbour patch dir world)))
 		((or (null patch) (eq (patch-biome patch) (get-biome 'stream))))
 		(setf (patch-biome patch) (get-biome 'stream))))
 
-(defun create-world (size &optional (world *world*))
-	(setf world NIL)
-	(setf world (init-matrix size))
-	;;XXX magic numbers
-	(generate-biomes 20 world)
-	(dotimes (s (round (/ (expt size 2) 500)))
-		(generate-stream (random size) (random size) world)))
+(defun create-world (size)
+	(let ((world (init-matrix size)))
+		;;XXX magic numbers
+		(generate-biomes 20 world)
+		(dotimes (s (round (/ (expt size 2) 500)))
+			(generate-stream (random size) (random size) world))
+		world))