add binary tree l-system from wikipedia

binary_tree.ld

@@ -0,0 +1,48 @@
+& Example 2: fractal (binary) tree
+& from https://en.wikipedia.org/wiki/L-system
+box states = []
+
+fn push! () -> {
+  update! (states, append (_, turtle_state ()))
+}
+
+fn pop! () -> {
+  let state = do states > unbox > last
+  update! (states, butlast)
+  penup! ()
+  load_turtle_state! (state)
+}
+
+fn branch! () -> {
+  pc! (colors :olive)
+  pw! (4)
+  fd! (20)
+}
+
+fn one! {
+  (0) -> {
+    branch! ()
+  }
+  (n) -> {
+    one! (dec (n))
+    one! (dec (n))
+  }
+}
+
+fn zero! {
+  (0) -> branch! ()
+  (n) -> {
+    one! (dec (n))
+    push! ()
+    lt! (inv (10))
+    zero! (dec (n))
+    pop! ()
+    rt! (inv (10))
+    zero! (dec (n))
+  }
+}
+
+zero! (7)
+
+hideturtle! ()
+