Some tentative attempts at optimisation. This, actually, makes no

difference in itself - because the world is immutable, this change
won't work without a different mechanism for generating the world
itself.

src/mw_engine/drainage.clj

@@ -35,13 +35,14 @@
    Flow comes from a higher cell to a lower only if the lower is the lowest neighbour of the higher."
    [world cell]
    (cond
-    (> (or (:altitude cell) 0) *sealevel*)
+     (not (nil? (:flow cell))) cell
+     (<= (or (:altitude cell) 0) *sealevel*) cell
+     true
       (merge cell
            {:flow (+ (:rainfall cell)
                (apply +
                  (map (fn [neighbour] (:flow (flow world neighbour)))
-                      (flow-contributors world cell))))})
-    true cell))
+                      (flow-contributors world cell))))})))
 
 (defn flow-world
   "Return a world like this `world`, but with cells tagged with the amount of