Create a pointer from Prop to Node.

src/nrnoc/section.h

@@ -225,17 +225,20 @@ struct Node {
 #include "hocdec.h" /* Prop needs Datum and Datum needs Symbol */
 #endif
 
+
 #define PROP_PY_INDEX 10
 struct Prop {
     // Working assumption is that we can safely equate "Prop" with "instance
     // of a mechanism" apart from a few special cases like CABLESECTION
-    Prop(short type)
-        : _type{type} {
+    Prop(Node* node, short type)
+        : node(node)
+        , _type{type} {
         if (type != CABLESECTION) {
             m_mech_handle = neuron::container::Mechanism::owning_handle{
                 neuron::model().mechanism_data(type)};
         }
     }
+    Node* node;      /* The node this property belongs to. */
     Prop* next;      /* linked list of properties */
     short _type;     /* type of membrane, e.g. passive, HH, etc. */
     int dparam_size; /* for notifying hoc_free_val_array */

src/nrnoc/treeset.cpp

@@ -675,7 +675,7 @@ Prop* prop_alloc(Prop** pp, int type, Node* nd) {
     nrn_alloc_node_ = nd;  // this might be null
     v_structure_change = 1;
     current_prop_list = pp;
-    auto* p = new Prop{static_cast<short>(type)};
+    auto* p = new Prop{nd, static_cast<short>(type)};
     p->next = *pp;
     p->ob = nullptr;
     p->_alloc_seq = -1;