cover non-noetherian case

monoid_homology/crs.py

@@ -190,7 +190,7 @@ def _reduce(self, word):
                 word = word.replace(left, right)
             if word == old:
                 return word
-        raise RuntimeError(f"No fixed point was found for {word0}"
+        raise RuntimeError(f"No fixed point was found for {word0} "
                            f"after {self.max_rewrites} iterations")
 
     def reduce(self, word):

test/test_monoid_homology.py

@@ -19,6 +19,8 @@
     kb_complete
 )
 
+import pytest
+
 def test_crs_homology():
     # Uses Sympy because it's hard to set up CI for SAGE.
 
@@ -193,3 +195,9 @@ def test_kb_eliminate_redundant_generators():
     # The example that caught the bug
     find_best_gens_crs([[0, 0, 0], [0, 0, 1], [0, 1, 0]], maxdim=4, extra=2)
 
+def test_non_noetherian():
+    crs = CRS("abc", [('cca', 'aca'), ('ba', 'bbc'), ('bca', 'cac')], max_rewrites=10_000)
+    culprit = "bcbcbcca"
+    msg = f"No fixed point was found for {culprit} after 10000 iterations"
+    with pytest.raises(RuntimeError, match=msg):
+        crs.reduce(culprit)