add doctest

Author: DaveWitteMorris

src/sage/combinat/designs/incidence_structures.py

@@ -1592,6 +1592,13 @@ def is_t_design(self, t=None, v=None, k=None, l=None, return_parameters=False):
             sage: I = IncidenceStructure(2, [[0],[0,1]])
             sage: I.is_t_design(return_parameters=True)
             (False, (0, 0, 0, 0))
+
+        Verify that :issue:`38454` is fixed::
+
+            sage: I = IncidenceStructure(points=[0,1,2,3,4,5],
+            ....:                        blocks=[[0,1], [1,2], [0,2]])
+            sage: I.is_t_design(return_parameters=True)
+            (True, (0, 6, 2, 3))
         """
         from sage.arith.misc import binomial