## Ruleset which attempts to model retreat of ice after an iceage

;; Limitations: because the rule language doesn't (yet) allow sophisticated
;; arithmetic, the ice retreats north to south (southern hemisphere). Otherwise,
;; it's pretty realistic; ice moves progressively up hillsides, and vegetation
;; gradually re-establishes.

## Vegetation rules
;; rules which populate the world with plants

;; Occasionally, passing birds plant tree seeds into grassland 

if state is grassland then 1 chance in 10 state should be heath

;; heath below the treeline grows gradually into forest, providing browsing pressure is not to high

if state is heath and fertility is more than 10 and altitude is less than 120 then state should be scrub 
if state is scrub and fertility is more than 20 then 1 chance in 20 state should be forest

;; Forest on fertile land grows to climax

if state is forest and fertility is more than 30 and altitude is less than 70 then state should be climax   
    
;; Climax forest occasionally catches fire (e.g. lightning strikes)

if state is climax then 1 chance in 500 state should be fire

;; Climax forest neighbouring fires is likely to catch fire
if state is climax and some neighbours are fire then 1 chance in 3 state should be fire

;; After fire we get waste

if state is fire then state should be waste

;; And after waste we get pioneer species; if there's a woodland seed
;; source, it's going to be heath, otherwise grassland.

if state is waste and some neighbours are scrub then state should be heath
if state is waste and some neighbours are forest then state should be heath
if state is waste and some neighbours are climax then state should be heath
if state is waste then state should be grassland

## Potential blockers

;; Woody plants increase soil fertility over time. 
if state is in heath or scrub or forest or climax then fertility should be fertility + 1


## Initialisation rules

;; Rules which deal with state 'new' will waste less time if they're near the
;; end of the file

if state is new then state should be ice

;; thaw moves gradually southwards (the distinction between 'ice' and 'snow' is
;; just a hack because I can't do complex arithmetic in rules)
;; below the waterline ice thaws to water.
if state is ice and generation is more than y and altitude is less than 10 then state should be water

;; otherwise it thaws to snow
if state is ice and generation is more than y then state should be snow

;; thaw moves gradually up the hills
if state is snow and generation is more than altitude then state should be waste
