diff --git a/project.clj b/project.clj
index 21b692e..e21d537 100644
--- a/project.clj
+++ b/project.clj
@@ -1,4 +1,4 @@
-(defproject mw-engine "0.1.6-SNAPSHOT"
+(defproject mw-engine "0.2.0-SNAPSHOT"
   :dependencies [[org.clojure/clojure "1.11.1"]
                  [org.clojure/clojurescript "1.11.60" :scope "provided"]
                  [org.clojure/math.combinatorics "0.2.0"]
diff --git a/src/cljc/mw_engine/core.clj b/src/cljc/mw_engine/core.clj
index 1483c57..e598f60 100644
--- a/src/cljc/mw_engine/core.clj
+++ b/src/cljc/mw_engine/core.clj
@@ -1,6 +1,27 @@
-(ns ^{:doc "Functions to transform a world and run rules."
+(ns ^{:doc "Functions to transform a world and run rules.
+            
+            Every rule is a function of two arguments, a cell and a world. If the rule
+            fires, it returns a new cell, which should have the same values for `:x` and
+            `:y` as the old cell. Anything else can be modified.
+
+            While any function of two arguments can be used as a rule, a special high
+            level rule language is provided by the `mw-parser` package, which compiles
+            rules expressed in a subset of English rules into suitable functions.
+
+            A cell is a map containing at least values for the keys :x, :y, and :state;
+            a transformation should not alter the values of :x or :y, and should not
+            return a cell without a keyword as the value of :state. Anything else is
+            legal.
+
+            A world is a two dimensional matrix (sequence of sequences) of cells, such
+            that every cell's `:x` and `:y` properties reflect its place in the matrix.
+            See `world.clj`.
+
+            Each time the world is transformed (see `transform-world`, for each cell,
+            rules are applied in turn until one matches. Once one rule has matched no
+            further rules can be applied to that cell."
       :author "Simon Brooke"}
-  mw-engine.core
+ mw-engine.core
   (:require [mw-engine.utils :refer [get-int-or-zero map-world]]
             [taoensso.timbre :as l]))
 
@@ -26,29 +47,6 @@
 ;;;; Copyright (C) 2014 Simon Brooke
 ;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;;;;
-;;;; Every rule is a function of two arguments, a cell and a world. If the rule
-;;;; fires, it returns a new cell, which should have the same values for :x and
-;;;; :y as the old cell. Anything else can be modified.
-;;;;
-;;;; While any function of two arguments can be used as a rule, a special high
-;;;; level rule language is provided by the `mw-parser` package, which compiles
-;;;; rules expressed in a subset of English rules into suitable functions.
-;;;;
-;;;; A cell is a map containing at least values for the keys :x, :y, and :state;
-;;;; a transformation should not alter the values of :x or :y, and should not
-;;;; return a cell without a keyword as the value of :state. Anything else is
-;;;; legal.
-;;;;
-;;;; A world is a two dimensional matrix (sequence of sequences) of cells, such
-;;;; that every cell's :x and :y properties reflect its place in the matrix.
-;;;; See `world.clj`.
-;;;;
-;;;; Each time the world is transformed (see `transform-world`, for each cell,
-;;;; rules are applied in turn until one matches. Once one rule has matched no
-;;;; further rules can be applied.
-;;;;
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (defn apply-rule
   "Apply a single `rule` to a `cell`. What this is about is that I want to be able,
@@ -62,19 +60,18 @@
      (ifn? rule) (apply-rule world cell rule nil)
      (seq? rule) (let [[afn src] rule] (apply-rule world cell afn src))))
   ([world cell rule source]
-    (let [result (apply rule (list cell world))]
-      (cond
-        (and result source) (merge result {:rule source})
-        :else result))))
+   (let [result (apply rule (list cell world))]
+     (cond
+       (and result source) (merge result {:rule source})
+       :else result))))
 
 (defn- apply-rules
   "Derive a cell from this `cell` of this `world` by applying these `rules`."
   [world cell rules]
   (cond (empty? rules) cell
-    :else (let [result (apply-rule world cell (first rules))]
-           (cond result result
-             :else (apply-rules world cell (rest rules))))))
-
+        :else (let [result (apply-rule world cell (first rules))]
+                (cond result result
+                      :else (apply-rules world cell (rest rules))))))
 
 (defn- transform-cell
   "Derive a cell from this `cell` of this `world` by applying these `rules`. If an
@@ -82,8 +79,8 @@
   [world cell rules]
   (try
     (merge
-      (apply-rules world cell rules)
-      {:generation (+ (get-int-or-zero cell :generation) 1)})
+     (apply-rules world cell rules)
+     {:generation (+ (get-int-or-zero cell :generation) 1)})
     (catch Exception e
       (merge cell {:error
                    (format "%s at generation %d when in state %s"
@@ -96,7 +93,7 @@
 (defn transform-world
   "Return a world derived from this `world` by applying these `rules` to each cell."
   ([world rules]
-  (map-world world transform-cell (list rules))))
+   (map-world world transform-cell (list rules))))
 
 (defn run-world
   "Run this world with these rules for this number of generations.
@@ -111,8 +108,8 @@
   (reduce (fn [world iteration]
             (l/info "Running iteration " iteration)
             (transform-world world rules))
-        (transform-world world init-rules)
-        (range generations)))
+          (transform-world world init-rules)
+          (range generations)))
 
 
 
diff --git a/src/cljc/mw_engine/display.clj b/src/cljc/mw_engine/display.clj
index d0e48a3..4a52445 100644
--- a/src/cljc/mw_engine/display.clj
+++ b/src/cljc/mw_engine/display.clj
@@ -35,18 +35,15 @@
   [state]
   (subs (str state) 1))
 
-
 (defn format-image-path
   "Render this `state`, assumed to be a keyword indicating a state in the
    world, into a path which should recover the corresponding image file."
   [state]
   (format "%s/%s.png" *image-base* (format-css-class state)))
 
-
 (defn format-mouseover [cell]
   (str cell))
 
-
 (defn render-cell
   "Render this world cell as a Hiccup table cell."
   [cell]
@@ -55,13 +52,11 @@
      [:a {:href (format "inspect?x=%d&y=%d" (:x cell) (:y cell))}
       [:img {:alt (:state cell) :width 32 :height 32 :src (format-image-path state)}]]]))
 
-
 (defn render-world-row
   "Render this world `row` as a Hiccup table row."
   [row]
   (apply vector (cons :tr (map render-cell row))))
 
-
 (defn render-world-table
   "Render this `world` as a Hiccup table."
   [world]
diff --git a/src/cljc/mw_engine/flow.clj b/src/cljc/mw_engine/flow.clj
index 778d8a7..78bff33 100644
--- a/src/cljc/mw_engine/flow.clj
+++ b/src/cljc/mw_engine/flow.clj
@@ -1,6 +1,26 @@
 (ns mw-engine.flow
-  "Allow flows of values between cells in the world."
-  (:require [mw-engine.utils :refer [get-cell get-num merge-cell]]
+  "Allow flows of values between cells in the world.
+   
+   The design here is: a flow object is a map with the following properties:
+   1. :source, whose value is a location;
+   2. :destination, whose value is a location;
+   3. :property, whose value is a keyword;
+   4. :quantity, whose value is a positive real number.
+
+   A location object is a map with the following properties:
+   1. :x, whose value is a natural number not greater than the extent of the world;
+   2. :y, whose value is a natural number not greater than the extent of the world.
+
+   To execute a flow is transfer the quantity specified of the property specified
+   from the cell at the source specified to the cell at the destination specified;
+   if the source doesn't have sufficient of the property, then all it has should
+   be transferred, but no more: properties to be flowed cannot be pulled negative.
+   
+   Flowing values through the world is consequently a two stage process: firstly
+   there's a planning stage, in which all the flows to be executed are computed
+   without changing the world, and then an execution stage, where they're all 
+   executed. This namespace deals with mainly with execution."
+  (:require [mw-engine.utils :refer [get-cell get-num in-bounds? merge-cell]]
             [taoensso.timbre :refer [info warn]]))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -25,32 +45,6 @@
 ;;;; Copyright (C) 2014 Simon Brooke
 ;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;;;;
-;;;; Functions to create and to print two dimensional cellular automata.
-;;;; Nothing in this namespace should determine what states are possible within
-;;;; the automaton, except for the initial state, :new.
-;;;;
-;;;; A cell is a map containing at least values for the keys :x, :y, and :state.
-;;;;
-;;;; A world is a two dimensional matrix (sequence of sequences) of cells, such
-;;;; that every cell's :x and :y properties reflect its place in the matrix.
-;;;;
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-
-;; OK, the design here is: a flow object is a map with the following properties:
-;; 1. :source, whose value is a location;
-;; 2. :destination, whose value is a location;
-;; 3. :property, whose value is a keyword;
-;; 4. :quantity, whose value is a positive real number.
-;; 
-;; A location object is a map with the following properties:
-;; 1. :x, whose value is a natural number not greater than the extent of the world;
-;; 2. :y, whose value is a natural number not greater than the extent of the world.
-;;
-;; to execute a flow is transfer the quantity specified of the property specified
-;; from the cell at the source specified to the cell at the destination specified;
-;; if the source doesn't have sufficient of the property, then all it has should
-;; be transferred, but no more. 
 
 (defn coordinate?
   "Return `true` if this object `o` is a valid coordinate with respect to
@@ -65,12 +59,13 @@
 
 (defn location?
   "Return `true` if this object `o` is a location as defined above with respect to
-   this `world`, else `false`. Assumes square worlds."
+   this `world`, else `false`."
   [o world]
   (try
     (and (map? o)
-         (coordinate? (:x o) world)
-         (coordinate? (:y o) world))
+         (integer? (:x o))
+         (integer? (:y o))
+         (in-bounds? world (:x o) (:y o)))
     (catch Exception e
       (warn (format "Not a valid location: %s; %s" o (.getMessage e)))
       false)))
@@ -98,16 +93,16 @@
     (let [sx (-> flow :source :x)
           sy (-> flow :source :y)
           source (get-cell world sx sy)
-        dx (-> flow :destination :x)
+          dx (-> flow :destination :x)
           dy (-> flow :destination :y)
           dest (get-cell world dx dy)
-        p (:property flow)
-        q (min (:quantity flow) (get-num source p))
-        s' (assoc source p (- (source p) q))
-        d' (assoc dest p (+ (get-num dest p) q))]
+          p (:property flow)
+          q (min (:quantity flow) (get-num source p))
+          s' (assoc source p (- (source p) q))
+          d' (assoc dest p (+ (get-num dest p) q))]
       (info (format "Moving %f units of %s from %d,%d to %d,%d"
                     (float q) (name p) sx sy dx dy))
-    (merge-cell (merge-cell world s') d'))
+      (merge-cell (merge-cell world s') d'))
     (catch Exception e
       (warn (format "Failed to execute flow %s: %s" flow (.getMessage e)))
       ;; return the world unmodified.
diff --git a/src/cljc/mw_engine/heightmap.clj b/src/cljc/mw_engine/heightmap.clj
index e282dcd..24c250d 100644
--- a/src/cljc/mw_engine/heightmap.clj
+++ b/src/cljc/mw_engine/heightmap.clj
@@ -1,4 +1,7 @@
-(ns ^{:doc "Functions to apply a heightmap to a world."
+(ns ^{:doc "Functions to apply a heightmap to a world.
+            
+            Heightmaps are considered only as greyscale images, so colour is redundent
+            (will be ignored). Darker shades are higher."
       :author "Simon Brooke"}
   mw-engine.heightmap
   (:require [mikera.image.core :refer [load-image filter-image]]
@@ -28,11 +31,6 @@
 ;;;; Copyright (C) 2014 Simon Brooke
 ;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;;;;
-;;;; Heightmaps are considered only as greyscale images, so colour is redundent
-;;;; (will be ignored). Darker shades are higher.
-;;;;
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (defn tag-property
   "Set the value of this `property` of this cell from the corresponding pixel of this `heightmap`.
diff --git a/src/cljc/mw_engine/natural_rules.clj b/src/cljc/mw_engine/natural_rules.clj
index b655704..f695409 100644
--- a/src/cljc/mw_engine/natural_rules.clj
+++ b/src/cljc/mw_engine/natural_rules.clj
@@ -1,5 +1,10 @@
-(ns ^{:doc "A set of MicroWorld rules describing a simplified natural ecosystem."
-       :author "Simon Brooke"}
+(ns ^{:doc "A set of MicroWorld rules describing a simplified natural ecosystem.
+
+            Since the completion of the rule language this is more or less obsolete -
+            there are still a few things that you can do with rules written in Clojure
+            that you can't do in the rule language, but not many and I doubt they're
+            important. "
+       :author " Simon Brooke "}
   mw-engine.natural-rules
   (:require [mw-engine.utils :refer [get-int get-neighbours get-neighbours-with-state member?]]))
 
@@ -25,13 +30,6 @@
 ;;;; Copyright (C) 2014 Simon Brooke
 ;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;;;;
-;;;; Since the completion of the rule language this is more or less obsolete -
-;;;; there are still a few things that you can do with rules written in Clojure
-;;;; that you can't do in the rule language, but not many and I doubt they're
-;;;; important.
-;;;;
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 ;; treeline at arbitrary altitude.
 (def treeline 150)
@@ -183,4 +181,4 @@
                     (list
                      vegetation-rules
                      herbivore-rules
-                     predator-rules)))
+                     predator-rules)))
\ No newline at end of file
diff --git a/src/cljc/mw_engine/utils.clj b/src/cljc/mw_engine/utils.clj
index 2254f77..1be5a61 100644
--- a/src/cljc/mw_engine/utils.clj
+++ b/src/cljc/mw_engine/utils.clj
@@ -47,7 +47,6 @@
   [_ cell]
   (merge cell {:generation (get-int-or-zero cell :generation)}))
 
-
 (defn in-bounds
   "True if x, y are in bounds for this world (i.e., there is a cell at x, y)
    else false. *DEPRECATED*: it's a predicate, prefer `in-bounds?`.
diff --git a/src/cljc/mw_engine/world.clj b/src/cljc/mw_engine/world.clj
index 34d1e1b..b5fcd17 100644
--- a/src/cljc/mw_engine/world.clj
+++ b/src/cljc/mw_engine/world.clj
@@ -1,8 +1,16 @@
-(ns ^{:doc "Functions to create and to print two dimensional cellular automata."
-       :author "Simon Brooke"}
-  mw-engine.world
-	(:require [clojure.string :as string]
-           [mw-engine.utils :refer [population]]))
+(ns ^{:doc "Functions to create and to print two dimensional cellular automata.
+            
+            Nothing in this namespace should determine what states are possible within
+            the automaton, except for the initial state, :new.
+
+            A cell is a map containing at least values for the keys `:x`, `:y`, and `:state`.
+
+            A world is a two dimensional matrix (sequence of sequences) of cells, such
+            that every cell's `:x` and `:y` properties reflect its place in the matrix."
+      :author "Simon Brooke"}
+ mw-engine.world
+  (:require [clojure.string :as string]
+            [mw-engine.utils :refer [population]]))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;
@@ -26,51 +34,14 @@
 ;;;; Copyright (C) 2014 Simon Brooke
 ;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;;;;
-;;;; Functions to create and to print two dimensional cellular automata.
-;;;; Nothing in this namespace should determine what states are possible within
-;;;; the automaton, except for the initial state, :new.
-;;;;
-;;;; A cell is a map containing at least values for the keys :x, :y, and :state.
-;;;;
-;;;; A world is a two dimensional matrix (sequence of sequences) of cells, such
-;;;; that every cell's :x and :y properties reflect its place in the matrix.
-;;;;
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-
-(defn- make-cell
+(defmacro make-cell
   "Create a minimal default cell at x, y
 
   * `x` the x coordinate at which this cell is created;
   * `y` the y coordinate at which this cell is created."
   [x y]
-  {:x x :y y :state :new})
-
-
-(defn- make-world-row
-  "Make the (remaining) cells in a row at this height in a world of this width.
-
-  * `index` x coordinate of the next cell to be created;
-  * `width` total width of the matrix, in cells;
-  * `height` y coordinate of the next cell to be created."
-  [index width height]
-  (cond (= index width) nil
-    :else (cons (make-cell index height)
-               (make-world-row (inc index) width height))))
-
-
-(defn- make-world-rows
-  "Make the (remaining) rows in a world of this width and height, from this
-   index.
-
-  * `index` y coordinate of the next row to be created;
-  * `width` total width of the matrix, in cells;
-  * `height` total height of the matrix, in cells."
-  [index width height]
-  (cond (= index height) nil
-    :else (cons (apply vector (make-world-row 0 width index))
-               (make-world-rows (inc index) width height))))
+  `{:x ~x :y ~y :state :new})
 
 (defn make-world
   "Make a world width cells from east to west, and height cells from north to
@@ -79,16 +50,17 @@
   * `width` a natural number representing the width of the matrix to be created;
   * `height` a natural number representing the height of the matrix to be created."
   [width height]
-  (apply vector (make-world-rows 0 width height)))
-
+  (apply vector
+         (map (fn [h]
+                (apply vector (map #(make-cell % h) (range width))))
+              (range height))))
 
 (defn truncate-state
   "Truncate the print name of the state of this cell to at most limit characters."
   [cell limit]
   (let [s (:state cell)]
     (cond (> (count (str s)) limit) (subs s 0 limit)
-      :else s)))
-
+          :else s)))
 
 (defn format-cell
   "Return a formatted string summarising the current state of this cell."
@@ -98,13 +70,11 @@
           (population cell :deer)
           (population cell :wolves)))
 
-
 (defn- format-world-row
   "Format one row in the state of a world for printing."
   [row]
   (string/join (map format-cell row)))
 
-
 (defn print-world
   "Print the current state of this world, and return nil.
 
@@ -112,8 +82,8 @@
   [world]
   (println)
   (dorun
-    (map
-      #(println
-         (format-world-row %))
-      world))
+   (map
+    #(println
+      (format-world-row %))
+    world))
   nil)