Make it easier to distinguish ThetaLarge matrices

python/dune/perftool/sumfact/amatrix.py

@@ -72,7 +72,7 @@ class LargeAMatrix(ImmutableRecord):
     def name(self):
         name = "ThetaLarge{}{}_{}_qp{}".format("face{}_".format(self.face) if self.face is not None else "",
                                                "T" if self.transpose else "",
-                                               "_".join(tuple("d" if d else "" for d in self.derivative)),
+                                               "x".join(tuple("d" if d else "" for d in self.derivative)),
                                                quadrature_points_per_direction(),
                                                )
         for i, d in enumerate(self.derivative):