diff --git a/resources/doc/markdown.css b/resources/doc/markdown.css
index 39629d1..3ddc079 100644
--- a/resources/doc/markdown.css
+++ b/resources/doc/markdown.css
@@ -2,8 +2,8 @@
   -fx-line-spacing: 4;
   -fx-font-family: Ubuntu;
   -fx-font-size: 14;
-  -fx-text-fill: #ddd;
-  -fx-fill: #ddd;
+  -fx-text-fill: black;
+  -fx-fill: black; 
 }
 
 .root {
diff --git a/resources/doc/mw-parser.md b/resources/doc/mw-parser.md
index 76f662d..29be2ae 100644
--- a/resources/doc/mw-parser.md
+++ b/resources/doc/mw-parser.md
@@ -54,7 +54,7 @@ all cells have x more than -1.
 If you are having problems because one of your rules isn't working, look to
 see whether there is another rule above it which is 'blocking' it.
 
-### <a name="grammar"></a>Grammar
+### Grammar
 
 #### Comments
 
@@ -77,7 +77,7 @@ In rules, _conditions_ is one of:
 + _condition_ and _conditions_
 + _condition_ or _conditions_
 
-Note that 'and' takes precedence over or, so
+Note that `and` takes precedence over `or`, so
 
     conditionA and conditionB or conditionC and conditionD
 
@@ -214,3 +214,5 @@ Copyright © 2014 [Simon Brooke](mailto:simon@journeyman.cc)
 
 Distributed under the terms of the
 [GNU General Public License v2](http://www.gnu.org/licenses/gpl-2.0.html)
+
+-----
\ No newline at end of file
diff --git a/src/mw_desktop/fxui.clj b/src/mw_desktop/fxui.clj
index f12fb97..ba03f66 100644
--- a/src/mw_desktop/fxui.clj
+++ b/src/mw_desktop/fxui.clj
@@ -1,9 +1,9 @@
 (ns mw-desktop.fxui
   (:require [cljfx.api :as fx]
-            [clojure.core.cache :refer [lru-cache-factory]]
-            [clojure.java.io :refer [resource]]
-            [clojure.string :refer [join lower-case starts-with?]]
-            [mw-desktop.state :refer [get-state state update-state!]]))
+            [mw-desktop.state :refer [state]]
+            [mw-desktop.ui-components.documentation-browser :refer [doc-browser]]
+            [mw-desktop.ui-components.rule-editor :refer [rule-editor]]
+            [mw-desktop.ui-components.world-view :refer [world-view]]))
 
 ;; OK, the basic idea here is we have a window divided vertically 
 ;; into two panes. The user can drag the division between the panes 
@@ -52,43 +52,35 @@
 ;; 
 ;; In which case you probably have one graph page per rule.
 
-(defn- tile-image [{:keys [url]}]
-  {:fx/type :image-view
-   :image {:url url
-           :requested-width 20
-           :preserve-ratio true
-           :background-loading true}})
-
-(defn world-view [{:keys [world tileset]}]
-  ;; assumes that by the time we get here, a tileset is a clojure map
-  ;; in which the keys are the names of the tiles, without file extension, as
-  ;; keywords (i.e. they're states, from the point of view of the world), and
-  ;; in which the values are just java images (bitmaps), or else maps which
-  ;; wrap java images with some other related data for example dimensions.
-  (let [th (or (:height (first tileset)) 20)
-        tw (or (:width (first tileset)) 20)
-        cols (count (first world))
-        rows (count world)]
-   {:fx/type :tile-pane
-    :hgap 0
-    :pref-columns cols
-    :pref-rows rows
-    :pref-tile-height th
-    :pref-tile-width tw
-    :vgap 0
-    :children (map (fn [cell]{:fx/type tile-image
-                      :tile-pane/alignment :bottom-center
-                      :url (resource (format "%s/%s.png" tileset (:state cell)))}) 
-                   (flatten world))}))
-
-(defn root-view [{{:keys [world rules]} :state}]
+(defn root-view [{{:keys []} :state}]
   {:fx/type :stage
    :showing true
+   :title "MicroWorld"
    :scene {:fx/type :scene
+           :stylesheets #{"doc/markdown.css"}
            :root {:fx/type :split-pane
-                  :items [{:fx.type :scroll-pane
-                           :content {:fx/type world-view}}]}}})
-
+                  :items [{:fx/type :stack-pane
+                           :children [{:fx/type :scroll-pane
+                                       :content {:fx/type world-view}}
+                                      {:fx/type :label
+                                       :stack-pane/alignment :top-right
+                                       :stack-pane/margin 5
+                                       :max-width 300
+                                      ;;  :text status
+                                       }]}
+                          {:fx/type :tab-pane
+                           :tabs [;; {:fx/type :tab
+                                  ;;  :text "Rule Editor"
+                                  ;;  :closable false
+                                  ;;  :content rule-editor
+                                  ;;  :parse-errors (or parse-errors "No errors found")
+                                  ;;  :rules-file (or rules-file "Unnamed file")
+                                  ;;  :rules-src (or rules-src ";; enter your rules here")
+                                  ;;  }
+                                  {:fx/type :tab
+                                   :text "Help"
+                                   :closable false
+                                   :content doc-browser}]}]}}})
 
 (defmulti handle-event :event/type)
 
@@ -98,7 +90,7 @@
 (defmethod handle-event ::type-text [{:keys [fx/event fx/context]}]
   {:context (fx/swap-context context assoc :typed-text event)})
 
-(defmulti event-handler 
+(defmulti event-handler
   "Multi-method event handler cribbed from e12-interactive-development"
   :event/type)
 
diff --git a/src/mw_desktop/io.clj b/src/mw_desktop/io.clj
index 69c960d..5111984 100644
--- a/src/mw_desktop/io.clj
+++ b/src/mw_desktop/io.clj
@@ -34,8 +34,9 @@
    Where a file and resource with this `path` both exist, the file is 
    preferred. Updates global state. Returns the ruleset loaded."
   [path]
-  (let [rules (doall (compile (slurp (:stream (identify-resource path)))))]
-    (update-state! :rules rules)
+  (let [src (slurp (:stream (identify-resource path)))
+        rules (doall (compile src))]
+    (update-state! :rules rules :rules-file path :rules-src src) 
     rules))
 
 (defn assemble-tile-set
@@ -93,7 +94,7 @@
                         path)
                        (merge data {:path path})
                        any))))]
-    (if (world? world) (do (update-state! :world world) world)
+    (if (world? world) (do (update-state! :world world :world-file path) world)
         (throw (ex-info "Invalid world file?"
                         (merge data {:path path
                                      :data world}))))))
diff --git a/src/mw_desktop/ui_components/documentation_browser.clj b/src/mw_desktop/ui_components/documentation_browser.clj
new file mode 100644
index 0000000..a19565d
--- /dev/null
+++ b/src/mw_desktop/ui_components/documentation_browser.clj
@@ -0,0 +1,260 @@
+(ns mw-desktop.ui-components.documentation-browser
+  "A tab in which documentation can be presented. Cribbed pretty much in
+   entirety from the cljfx example `e20-markdown-editor`."
+  (:require [cljfx.api :as fx]
+            [clojure.core.cache :refer [lru-cache-factory]]
+            [clojure.java.io :refer [resource]]
+            [clojure.string :as str]
+            [mw-desktop.state :refer [state]])
+  (:import [java.awt Desktop]
+           [java.io File]
+           [java.net URI]
+           [org.commonmark.node Node]
+           [org.commonmark.parser Parser]))
+
+(defn commonmark->clj [^Node node]
+  (let [tag (->> node
+                 .getClass
+                 .getSimpleName
+                 (re-seq #"[A-Z][a-z]+")
+                 (map str/lower-case)
+                 (str/join "-")
+                 keyword)
+        all-attrs (->> node
+                       bean
+                       (map (fn [[k v]]
+                              [(->> k
+                                    name
+                                    (re-seq #"[A-Z]?[a-z]+")
+                                    (map str/lower-case)
+                                    (str/join "-")
+                                    keyword)
+                               v]))
+                       (into {}))]
+    {:tag tag
+     :attrs (dissoc all-attrs :next :previous :class :first-child :last-child :parent)
+     :children (->> node
+                    .getFirstChild
+                    (iterate #(.getNext ^Node %))
+                    (take-while some?)
+                    (mapv commonmark->clj))}))
+
+(defn node-sub [context]
+  (-> (Parser/builder)
+      .build
+      (.parse (fx/sub-val context :typed-text))
+      commonmark->clj))
+
+(defmulti handle-event :event/type)
+
+(defmethod handle-event :default [e]
+  (prn e))
+
+(defmethod handle-event ::type-text [{:keys [fx/event fx/context]}]
+  {:context (fx/swap-context context assoc :typed-text event)})
+
+(defmulti md->fx :tag)
+
+(defn md-view [{:keys [node]}]
+  (md->fx node))
+
+(defmethod md->fx :heading [{children :children {:keys [level]} :attrs}]
+  {:fx/type :text-flow
+   :style-class ["heading" (str "level-" level)]
+   :children (for [node children]
+               {:fx/type md-view
+                :node node})})
+
+(defmethod md->fx :paragraph [{children :children}]
+  {:fx/type :text-flow
+   :style-class "paragraph"
+   :children (for [node children]
+               {:fx/type md-view
+                :node node})})
+
+(defmethod md->fx :text [{{:keys [literal]} :attrs}]
+  {:fx/type :text
+   :cache true
+   :cache-hint :speed
+   :text literal})
+
+(defmethod md->fx :code [{{:keys [literal]} :attrs}]
+  {:fx/type :label
+   :cache true
+   :cache-hint :speed
+   :style-class "code"
+   :text literal})
+
+(defmethod md->fx :fenced-code-block [{{:keys [literal]} :attrs}]
+  {:fx/type :v-box
+   :padding {:top 9}
+   :children [{:fx/type :scroll-pane
+               :style-class ["scroll-pane" "code-block"]
+               :fit-to-width true
+               :content {:fx/type :label
+                         :cache true
+                         :cache-hint :speed
+                         :max-width ##Inf
+                         :min-width :use-pref-size
+                         :text literal}}]})
+
+(defmethod md->fx :indented-code-block [{{:keys [literal]} :attrs}]
+  {:fx/type :v-box
+   :padding {:top 9}
+   :children [{:fx/type :scroll-pane
+               :style-class ["scroll-pane" "code-block"]
+               :fit-to-width true
+               :content {:fx/type :label
+                         :cache true
+                         :cache-hint :speed
+                         :max-width ##Inf
+                         :min-width :use-pref-size
+                         :text literal}}]})
+
+(defmethod md->fx :link [{{:keys [^String destination]} :attrs children :children}]
+  (let [link {:fx/type :hyperlink
+              :on-action (fn [_]
+                           (future
+                             (try
+                               (if (str/starts-with? destination "http")
+                                 (.browse (Desktop/getDesktop) (URI. destination))
+                                 (.open (Desktop/getDesktop) (File. destination)))
+                               (catch Exception e
+                                 (.printStackTrace e)))))}]
+    (if (and (= 1 (count children))
+             (= :text (:tag (first children))))
+      (assoc link :text (-> children first :attrs :literal))
+      (assoc link :graphic {:fx/type :h-box
+                            :children (for [node children]
+                                        {:fx/type md-view
+                                         :node node})}))))
+
+(defmethod md->fx :strong-emphasis [{:keys [children]}]
+  (if (and (= 1 (count children))
+           (= :text (:tag (first children))))
+    {:fx/type :text
+     :cache true
+     :cache-hint :speed
+     :style-class "strong-emphasis"
+     :text (-> children first :attrs :literal)}
+    {:fx/type :h-box
+     :cache true
+     :style-class "strong-emphasis"
+     :children (for [node children]
+                 {:fx/type md-view
+                  :node node})}))
+
+(defmethod md->fx :emphasis [{:keys [children]}]
+  (if (and (= 1 (count children))
+           (= :text (:tag (first children))))
+    {:fx/type :text
+     :cache true
+     :cache-hint :speed
+     :style-class "emphasis"
+     :text (-> children first :attrs :literal)}
+    {:fx/type :h-box
+     :style-class "emphasis"
+     :children (for [node children]
+                 {:fx/type md-view
+                  :node node})}))
+
+(defmethod md->fx :soft-line-break [_]
+  {:fx/type :text
+   :text " "})
+
+(defmethod md->fx :document [{:keys [children]}]
+  {:fx/type :v-box
+   :style-class "document"
+   :children (for [node children]
+               {:fx/type md-view
+                :node node})})
+
+(defmethod md->fx :image [{{:keys [destination]} :attrs}]
+  {:fx/type :image-view
+   :image {:url (if (str/starts-with? destination "http")
+                  destination
+                  (str "file:" destination))
+           :background-loading true}})
+
+(defmethod md->fx :bullet-list [{{:keys [bullet-marker]} :attrs children :children}]
+  {:fx/type :v-box
+   :style-class "md-list"
+   :children (for [node children]
+               {:fx/type :h-box
+                :alignment :baseline-left
+                :spacing 4
+                :children [{:fx/type :label
+                            :min-width :use-pref-size
+                            :cache true
+                            :cache-hint :speed
+                            :text (str bullet-marker)}
+                           {:fx/type md-view
+                            :node node}]})})
+
+(defmethod md->fx :ordered-list [{{:keys [delimiter start-number]} :attrs
+                                  children :children}]
+  {:fx/type :v-box
+   :style-class "md-list"
+   :children (map (fn [child number]
+                    {:fx/type :h-box
+                     :alignment :baseline-left
+                     :spacing 4
+                     :children [{:fx/type :label
+                                 :cache true
+                                 :cache-hint :speed
+                                 :min-width :use-pref-size
+                                 :text (str number delimiter)}
+                                (assoc (md->fx child)
+                                       :h-box/hgrow :always)]})
+                  children
+                  (range start-number ##Inf))})
+
+(defmethod md->fx :list-item [{:keys [children]}]
+  {:fx/type :v-box
+   :children (for [node children]
+               {:fx/type md-view
+                :node node})})
+
+(defmethod md->fx :default [{:keys [tag attrs children]}]
+  {:fx/type :v-box
+   :children [{:fx/type :label
+               :cache true
+               :cache-hint :speed
+               :style {:-fx-background-color :red}
+               :text (str tag " " attrs)}
+              {:fx/type :v-box
+               :padding {:left 10}
+               :children (for [node children]
+                           {:fx/type md-view
+                            :node node})}]})
+
+(def *context
+  (atom
+   (fx/create-context
+    {:typed-text (subs (slurp (resource "doc/mw-parser.md")) 0 10000)}
+    #(lru-cache-factory % :threshold 498))))
+
+(defn doc-browser
+  [{:keys [fx/context]}]
+  {:fx/type :scroll-pane
+   :fit-to-width true
+   :content {:fx/type md-view
+             :node (fx/sub-ctx context node-sub)}})
+
+(defmulti handle-event :event/type)
+
+(defmethod handle-event :default [e]
+  (prn e))
+
+(def app
+  "Test purposes only"
+  (fx/create-app *context
+                 :event-handler handle-event
+                 :desc-fn (fn [_]
+                            {:fx/type :stage
+                             :showing true
+                             :width 960
+                             :height 540
+                             :scene {:fx/type :scene
+                                     :stylesheets #{"doc/markdown.css"}
+                                     :root {:fx/type doc-browser}}})))
\ No newline at end of file
diff --git a/src/mw_desktop/ui_components/rule_editor.clj b/src/mw_desktop/ui_components/rule_editor.clj
new file mode 100644
index 0000000..3d8f396
--- /dev/null
+++ b/src/mw_desktop/ui_components/rule_editor.clj
@@ -0,0 +1,34 @@
+(ns mw-desktop.ui-components.rule-editor 
+  (:require [cljfx.api :as fx]
+            [clojure.string :refer [join]]
+            [mw-desktop.state :refer :all]))
+
+(defn rule-editor 
+  "Rule editor comprises a scrolling container, with within it,
+   
+   1. name of loaded rule file (non editable, visually distinguished) at the
+      top;
+   2. line numbers in a column on the left;
+   3. editable rules source text in the middle;
+   4. parser/compiler errors in a panel as the bottom."
+  [{{:keys [rules-file rules-src parse-errors]} :state}]
+  {:fx/type :scroll-pane
+   :content {:fx/type :border-pane
+   :top {:fx/type :label
+         :border-pane/alignment :center
+         :border-pane/margin 2
+         :text rules-file}
+   :left {:fx/type :label
+          :border-pane/margin 2
+          :text (join "\n" 
+                      (map #(format "%5d" %) 
+                           (range 1 (inc (count (re-seq #"\n" rules-src))))))}
+   :center {:fx/type :text-area
+            :style-class "input"
+            :text (fx/sub-val rules-src :typed-text)
+            :on-text-changed {:event/type ::type-text :fx/sync true}}
+   :bottom {:fx/type :label
+            :border-pane/margin 2
+            :text (or parse-errors "error messages will appear here.")}}
+   }
+  )
diff --git a/src/mw_desktop/ui_components/world_view.clj b/src/mw_desktop/ui_components/world_view.clj
new file mode 100644
index 0000000..66dd24c
--- /dev/null
+++ b/src/mw_desktop/ui_components/world_view.clj
@@ -0,0 +1,72 @@
+(ns mw-desktop.ui-components.world-view
+  "View of the world"
+  (:require [cljfx.api :as fx]
+            [clojure.java.io :refer [as-url resource]]
+            [mw-desktop.io :refer [load-world!]]
+            [mw-desktop.state :refer [state]]))
+
+;; World view is essentially a grid of tiles, one per cell. In the long term 
+;; I want a context menu, allowing the user to
+;;
+;; 1. inspect cell properties;
+;; 2. change non-protected properties, including state;
+;; 3. browse cell history in a user-friendly presentation.
+;;
+;; In the short term, just being able to scroll around the grid is enough.
+
+(defn- tile-image [{:keys [url]}]
+  {:fx/type :image-view
+   :image {:url url
+           :requested-width 20
+           :preserve-ratio true
+           :background-loading true}})
+
+(defn assemble-tiles
+  [world tiles]
+  (map (fn [cell]
+         {:fx/type tile-image
+          :tile-pane/alignment :bottom-center
+          :url (as-url (or (tiles (:state cell))
+                           (resource "tilesets/world/error.png")))})
+       (flatten world)))
+
+(defn world-view
+  [{{:keys [world tileset]} :state}]
+  ;; assumes that by the time we get here, a tileset is a clojure map
+  ;; in which the keys are the names of the tiles, without file extension, as
+  ;; keywords (i.e. they're states, from the point of view of the world), and
+  ;; in which the values are URLs which point to image files.
+  (let [th (or (:height (first tileset)) 20)
+        tw (or (:width (first tileset)) 20)
+        cols 48 ;; (count (first world))
+        rows 47;; (count world)
+        children (assemble-tiles world tileset)]
+    {:fx/type :tile-pane
+     :hgap 0
+     :pref-columns cols
+     :pref-rows rows
+     :pref-tile-height th
+     :pref-tile-width tw
+     :vgap 0
+     :children children}))
+
+
+;;; From this point on we're just constructing a test harness to launch the 
+;;; component in isolation.
+(load-world! "init-state/world.edn")
+(defmulti handle-event :event/type)
+
+(defmethod handle-event :default [e]
+  (prn e))
+
+(def app
+  "Test purposes only"
+  (fx/create-app state
+                 :event-handler handle-event
+                 :desc-fn (fn [_]
+                            {:fx/type :stage
+                             :showing true
+                             :width 960
+                             :height 540
+                             :scene {:fx/type :scene
+                                     :root {:fx/type world-view}}})))
\ No newline at end of file