Add comment on custom prefix

dune/common/parametertree.cc

@@ -20,6 +20,9 @@ using namespace Dune;
 ParameterTree::ParameterTree()
 {}
 
+// Since this (internal) tree is static and constant, its prefix cannot be changed even
+// though we report it as the sub-tree of another one (in `.sub(prefix) const`).
+// Thus, we set the prefix as "<unknown>" to inform users that we could not construct the real prefix.
 ParameterTree::ParameterTree(ParameterTree::EmptyTag)
 : prefix_{"<unknown>"}
 {}

dune/common/parametertree.hh

@@ -40,6 +40,9 @@ namespace Dune {
     template<typename T>
     struct Parser;
 
+    struct EmptyTag {};
+    ParameterTree(EmptyTag);
+
   public:
 
     /** \brief storage for key lists
@@ -207,9 +210,6 @@ namespace Dune {
 
   protected:
 
-    struct EmptyTag {};
-    ParameterTree(EmptyTag);
-
     static const ParameterTree empty_;
 
     std::string prefix_;