Do not implement differential twice

dune/tectonic/nicefunction.hh

@@ -81,12 +81,7 @@ class RuinaFunction : public NiceFunction {
 
   /* see above */
   double virtual rightDifferential(double s) const {
-    double const arg = eta * s / h;
-    if (arg <= rho)
-      return 0;
-
-    return coefficient * normalStress *
-           (a * std::log(arg) + mu + compound_state);
+    return leftDifferential(s);
   }
 
   /*