test RuntimeError no invertible matrix

Author: dkrenn

src/sage/combinat/k_regular_sequence.py

@@ -1239,6 +1239,14 @@ def guess(self, f, n_verify=100, max_exponent=10, sequence=None):
             2-regular sequence 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ...
             sage: S2
             2-regular sequence 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ...
+
+        ::
+
+            sage: A = Seq2((Matrix([[1, 1], [1, 1]]), Matrix([[1, 1], [1, 1]])), left=(1, 1), right=(1, 1))
+            sage: Seq2.guess(lambda n: n, sequence=A, n_verify=5)
+            Traceback (most recent call last):
+            ...
+            RuntimeError: no invertible submatrix found
         """
         import logging
         logger = logging.getLogger(__name__)