Rebuilt some sphinx docs.

docs/Trees.rst

@@ -768,7 +768,7 @@ to understand:
 .. code:: ipython2
 
     from joy.library import FunctionWrapper
-    from joy.utils.stack import pushback
+    from joy.utils.stack import concat
     from notebook_preamble import D
     
     
@@ -791,7 +791,7 @@ to understand:
                             L
         '''
         L, (E, (G, (b, (a, stack)))) = stack
-        expression = pushback(G if a > b else L if a < b else E, expression)
+        expression = concat(G if a > b else L if a < b else E, expression)
         return stack, expression, dictionary