add reverse, snippet for prepend implementation

src/segment.rs

@@ -126,6 +126,20 @@ impl Seg {
         }
     }
 
+    /* applies a function across a list in standard form
+     * recurs before applying function to go in reverse
+     * function must take a Ctr and return a bool
+     * short circuits on the first false returned.
+     * also returns false on a non standard form list
+     */
+    pub fn circuit_reverse<F: FnMut(&Ctr) -> bool>(&self, func: &mut F) -> bool {
+        match &*self.cdr {
+            Ctr::None => func(&self.car),
+            Ctr::Seg(l) => l.circuit_reverse(func) && func(&self.car),
+            _ => false,
+        }
+    }
+
     pub fn from_mono(arg: Box<Ctr>) -> Seg {
         Seg {
             car: arg,