Simplify mask construction to prevent unnecessary temporaries

mclevel.py

@@ -111,6 +111,8 @@ Copyright 2010 David Rio Vierra
 """
 
 import nbt
+import operator
+import functools
 from nbt import *
 import gzip
 import StringIO
@@ -462,10 +464,7 @@ class MCLevel(object):
         
         blocks = self.Blocks[slices[0],slices[2],slices[1]]
         if blocksToReplace != None:
-            masks = map(lambda x:blocks==x, blocksToReplace);
-            mask = masks.pop();
-            while len(masks):
-                mask |= masks.pop();
+            mask = functools.reduce((blocks==x for x in blocksToReplace), operator.or_)
                 
             blocks[mask] = blockType;
             if hasattr(self, "Data"):