diff --git a/python/ptolemy/processFileBase.py b/python/ptolemy/processFileBase.py
index f47f70e1..69cc395e 100644
--- a/python/ptolemy/processFileBase.py
+++ b/python/ptolemy/processFileBase.py
@@ -1,6 +1,7 @@
 import re
 
 from . import utilities
+from .ptolemyObstructionClass import PtolemyObstructionClass
 
 """
 Basic functions to read a ptolemy solutions file.
@@ -139,6 +140,47 @@ def get_py_eval(text):
         utilities.join_long_lines(
             find_unique_section(text, "PY=EVAL=SECTION")))
 
+# A dict-like object we can feed into the variable dict
+# function in the pre-computed solution file.
+#
+# It is sufficient to evaluate the obstruction class but
+# returns 1 for all other keys.
+#
+class _DummyDict:
+    def __getitem__(self, key):
+        if key == '1' or key[0] == 'c':
+            return 1
+        raise KeyError(key)
+
+def check_obstruction_class_for_variable_dict_function(
+        variable_dict_function, obstruction_class):
+
+    if not isinstance(obstruction_class, PtolemyObstructionClass):
+        # Note that no obstruction class has always been
+        # corresponding to index 0 ("c0" in file name).
+        # So nothing to check.
+        #
+        # We also don't check for the generalized
+        # obstruction class.
+        #
+        # We should double check that our solutions
+        # for PSL(n,C) with n>2 are still fine...
+        #
+        return
+
+    variable_dict = variable_dict_function(_DummyDict())
+    precomputed_class = [
+        variable_dict[var_name]
+        for var_name in obstruction_class._explain_basis ]
+
+    # Obstruction class contains element in Z/2 as additive group.
+    # We need to convert it to be in multiplicative group {-1, 1}.
+    actual_class = [
+        (-1) ** i for i in obstruction_class._H2_element ]
+
+    if actual_class != precomputed_class:
+        print("Warning: the obstruction class does not match the obstruction class of the pre-computed solution.")
+        print(actual_class, precomputed_class)
 
 def get_manifold_thunk(text):
     """
diff --git a/python/ptolemy/ptolemyVariety.py b/python/ptolemy/ptolemyVariety.py
index f6d2ef3e..f8e67714 100644
--- a/python/ptolemy/ptolemyVariety.py
+++ b/python/ptolemy/ptolemyVariety.py
@@ -495,7 +495,8 @@ def _solution_file_url(self, data_url=None, rur=False):
 
         return data_url + self.path_to_file() + '/' + urlquote(filename)
 
-    def _retrieve_solution_file(self, data_url=None, prefer_rur=False,
+    def _retrieve_solution_file(self,
+                                data_url=None, prefer_rur=False,
                                 verbose=False):
 
         # First try to retrieve solutions from the URL corresponding to
@@ -510,15 +511,45 @@ def _retrieve_solution_file(self, data_url=None, prefer_rur=False,
             return _retrieve_url(url)
 
         except PtolemyFileMissingError:
+            pass
 
-            # If that file wasn't there, try to retrieve solutions from URL
-            # corresponding to the non-prefered format
+        # If that file wasn't there, try to retrieve solutions from URL
+        # corresponding to the non-prefered format
 
-            url = self._solution_file_url(data_url=data_url,
-                                          rur=not prefer_rur)
-            if verbose:
-                print("Retrieving solutions instead from %s ...:" % url)
-            return _retrieve_url(url)
+        url = self._solution_file_url(data_url=data_url,
+                                      rur=not prefer_rur)
+        if verbose:
+            print("Retrieving solutions instead from %s ...:" % url)
+
+        return _retrieve_url(url)
+
+    def _check_obstruction_class_for_precomputed_file(
+            self, text):
+
+        # Grab the PY=EVAL=SECTION
+        # Note that this will be evaluated again later.
+        # But it is cheap enough that the double evaluation should not matter.
+        py_eval = processFileBase.get_py_eval(text)
+
+        # Now get the function that takes a dictionary-like object with
+        # the solutions to the variety and expands it to a dictionary
+        # assigning a value to all Ptolemy coordinates and obstruction class
+        # variables.
+        variable_dict_function = py_eval['variable_dict']
+
+        # As noticed by Nathan, the obstruction classes used for producing
+        # the pre-computed solutions do not match the obstruction classes
+        # we compute now.
+        # This is due to pari changing the code for computing the
+        # Smith normal form in version 2.13 (see
+        # http://pari.math.u-bordeaux.fr/archives/pari-announce-20/msg00006.html)
+        #
+        # We thus check here explicitly that the obstruction class still
+        # matches.
+        #
+        processFileBase.check_obstruction_class_for_variable_dict_function(
+            variable_dict_function,
+            self._obstruction_class)
 
     def retrieve_decomposition(self, data_url=None, verbose=True):
 
@@ -545,6 +576,9 @@ def retrieve_solutions(self, numerical=False,
         text = self._retrieve_solution_file(data_url=data_url,
                                             prefer_rur=prefer_rur,
                                             verbose=verbose)
+
+        self._check_obstruction_class_for_precomputed_file(text)
+
         if verbose:
             print("Parsing...")