From 2f7f20167ba6da5b6197b48333772d292662d2ef Mon Sep 17 00:00:00 2001
From: Isuru Fernando <isuruf@gmail.com>
Date: Tue, 3 Oct 2017 16:33:18 -0500
Subject: [PATCH] Help CSE by using UnevaluatedExpr

---
 sumpy/expansion/multipole.py |  4 ++--
 sumpy/symbolic.py            | 13 +++++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/sumpy/expansion/multipole.py b/sumpy/expansion/multipole.py
index 4e226cd9..92ae80f3 100644
--- a/sumpy/expansion/multipole.py
+++ b/sumpy/expansion/multipole.py
@@ -178,11 +178,11 @@ class VolumeTaylorMultipoleExpansionBase(MultipoleExpansionBase):
                     assert n >= k
                     from sympy import binomial
                     contrib *= (binomial(n, k)
-                            * dvec[idim]**(n-k))
+                            * sym.UnevaluatedExpr(dvec[idim]/tgt_rscale)**(n-k))
 
                 result[i] += (
                         contrib
-                        * (src_rscale**sum(src_mi) / tgt_rscale**sum(tgt_mi)))
+                        * sym.UnevaluatedExpr(src_rscale/tgt_rscale)**sum(src_mi))
 
             result[i] /= mi_factorial(tgt_mi)
 
diff --git a/sumpy/symbolic.py b/sumpy/symbolic.py
index c213f4b9..c7ed61e6 100644
--- a/sumpy/symbolic.py
+++ b/sumpy/symbolic.py
@@ -98,6 +98,19 @@ def _coeff_isneg(a):
     return a.is_Number and a.is_negative
 
 
+have_unevaluated_expr = False
+if not USE_SYMENGINE:
+    try:
+        from sympy import UnevaluatedExpr
+        have_unevaluated_expr = True
+    except ImportError:
+        pass
+
+if not have_unevaluated_expr:
+    def UnevaluatedExpr(x):  # noqa
+        return x
+
+
 # {{{ debugging of sympy CSE via Maxima
 
 class _DerivativeKiller(IdentityMapperBase):
-- 
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