diff --git a/dune/tectonic/globalnonlinearity.hh b/dune/tectonic/globalnonlinearity.hh
index 4ea304e0e0d5a2e6b8337413cc70e44a4e9df70e..b8c624928db99bceb6091e2f7b80f4b9785512d2 100644
--- a/dune/tectonic/globalnonlinearity.hh
+++ b/dune/tectonic/globalnonlinearity.hh
@@ -18,6 +18,15 @@ class GlobalNonlinearity {
   typedef VectorTypeTEMPLATE VectorType;
   typedef MatrixTypeTEMPLATE MatrixType;
 
+  double operator()(VectorType const &x) const {
+    double tmp = 0;
+    for (size_t i = 0; i < x.size(); ++i) {
+      auto const res = restriction(i);
+      tmp += (*res)(x[i]);
+    }
+    return tmp;
+  }
+
   /*
     Return a restriction of the outer function to the i'th node.
   */