2016-01-08: add critical rolls to d6

add critical rolls to d6

diff --git a/examples/d6.w b/examples/d6.w
--- a/examples/d6.w
+++ b/examples/d6.w
@@ -4,24 +4,37 @@
 define-module : examples d6
    . #:export : roll check
 
+use-modules : srfi srfi-1
+
 ; basic d6 rules, implemented in guile
 
 define : roll
     . "Roll one ± d6"
-    let : : die '(-5 -3 -1 2 4 6)
-        list-ref die : random 6 : random-state-from-platform
-
+    let* 
+        : eyes '(-5 -3 -1 2 4 6)
+          d6 : lambda () : list-ref eyes : random 6 : random-state-from-platform
+        let rolling : : rolled : cons (d6) '()
+            cond
+              : = 1 (length rolled)
+                if : not : member (first rolled) '(-5, 6)
+                     first rolled
+                     rolling : cons (d6) rolled
+              : not : equal? (first rolled) (second rolled)
+                apply + : cdr rolled
+              else
+                rolling : cons (d6) rolled
+                
+                
+          
 define : check skill target effect-threshold
     . "Check whether a given skill-roll succeeds and provide a margin of success."
     let : : result : + skill : roll
         if : > result target
-            if : < effect-threshold : - result target
-                . 1
-                . #t
+            floor/ {result - target} effect-threshold
             . #f
 
 display : check 12 9 3
 newline
 display : roll
+newline
 
-