suggested doctest added

Author: fchapoton

src/sage/matrix/matrix2.pyx

@@ -8542,15 +8542,18 @@ cdef class Matrix(Matrix1):
 
         TESTS::
 
+            sage: # needs sage.graphs
             sage: M = matrix(ZZ, [[3, 4, 5], [3, 4, 5], [3, 5, 4], [2, 0,1]])
-            sage: M.permutation_normal_form()                                           # needs sage.graphs
+            sage: M.permutation_normal_form()
             [5 4 3]
             [5 4 3]
             [4 5 3]
             [1 0 2]
             sage: M = matrix(ZZ, 0, 0, [])
-            sage: M.permutation_normal_form()                                           # needs sage.graphs
+            sage: M.permutation_normal_form()
             []
+            sage: M.permutation_normal_form(check=True)
+            ([], ((), ()))
         """
         nrows = self.nrows()
         ncols = self.ncols()