diff --git a/src/octTree.hpp b/src/octTree.hpp
index 6dd846d..14bf17d 100644
--- a/src/octTree.hpp
+++ b/src/octTree.hpp
@@ -47,12 +47,25 @@ namespace CosmoTool
       return cells[0].numberLeaves;
     }
 
+    static bool unconditioned(const FCoordinates&, octPtr, float, bool)
+    {
+      return true;
+    }
+
     template<typename FunT>
     void walkTree(FunT f)
     {
       OctCoords rootCenter = { octCoordCenter, octCoordCenter, octCoordCenter };
 
-      walkTreeElements(f, 0, rootCenter, octCoordCenter);
+      walkTreeElements(f, unconditioned, 0, rootCenter, octCoordCenter);
+    }
+
+    template<typename FunT, typename CondT>
+    void walkTree(FunT f, CondT condition)
+    {
+      OctCoords rootCenter = { octCoordCenter, octCoordCenter, octCoordCenter };
+
+      walkTreeElements(f, condition, 0, rootCenter, octCoordCenter);
     }
 
   protected:
@@ -66,8 +79,8 @@ namespace CosmoTool
     float xMin[3];
 
 
-    template<typename FunT>
-    void walkTreeElements(FunT f, octPtr node, 
+    template<typename FunT,typename CondT>
+    void walkTreeElements(FunT f, CondT condition, octPtr node, 
 			  const OctCoords& icoord,
 			  octCoordType halfNodeLength)
     {
@@ -79,11 +92,16 @@ namespace CosmoTool
 	  center[j] = icoord[j]/(2.*octCoordCenter);
 	  realCenter[j] = xMin[j] + center[j]*lenNorm;
 	}
-      
+
       f(realCenter, cells[node].numberLeaves, lenNorm*halfNodeLength/(float)octCoordCenter,
 	cells[node].children[0] == invalidOctCell, // True if this is a meta-node
 	false);
-      
+     
+      if (!condition(realCenter, cells[node].numberLeaves,
+		     lenNorm*halfNodeLength/(float)octCoordCenter,
+		     cells[node].children[0] == invalidOctCell))
+	return;
+       
       for (int i = 0; i < 8; i++)
 	{
 	  octPtr newNode = cells[node].children[i];
@@ -108,7 +126,7 @@ namespace CosmoTool
 		false, true);
 	      continue;
 	    }
-	  walkTreeElements(f, cells[node].children[i], newCoord, halfNodeLength/2);
+	  walkTreeElements(f, condition, cells[node].children[i], newCoord, halfNodeLength/2);
 	}
       
     }