Test that M2L(0) = 0

diff --git a/boxtree/pyfmmlib_integration.py b/boxtree/pyfmmlib_integration.py
index a32d1e6..4577565 100644
--- a/boxtree/pyfmmlib_integration.py
+++ b/boxtree/pyfmmlib_integration.py
@@ -580,6 +580,8 @@ class FMMLibExpansionWrangler(object):
 
         mploc = self.get_translation_routine("%ddmploc", vec_suffix="_imany")
 
+        mpole_exps = 0 * mpole_exps
+
         for lev in range(self.tree.nlevels):
             lstart, lstop = level_start_target_or_target_parent_box_nrs[lev:lev+2]
             if lstart == lstop:
@@ -651,6 +653,8 @@ class FMMLibExpansionWrangler(object):
 
                     **kwargs).T
 
+            print("expn2 norm", np.linalg.norm(expn2.flatten()))
+
             target_local_exps_view[tgt_ibox_vec - target_level_start_ibox] += expn2
 
         return local_exps