Implement order assignment to functional events

Event trees manage the order of functional events.
The order implicitly deduced
from the order of functional event addition into event trees.

Issue #150

src/event_tree.cc

@@ -41,9 +41,12 @@ void EventTree::Add(SequencePtr sequence) {
 }
 
 void EventTree::Add(FunctionalEventPtr functional_event) {
+  assert(functional_event->order() == 0 && "Non-unique functional event.");
+  auto& unordered_event = *functional_event;
   mef::AddElement<ValidationError>(std::move(functional_event),
                                    &functional_events_,
                                    "Duplicate functional event: ");
+  unordered_event.order(functional_events_.size());
 }
 
 void EventTree::Add(NamedBranchPtr branch) {

src/event_tree.h

@@ -82,10 +82,24 @@ class Sequence : public Element {
 /// Sequences are defined in event trees but referenced in other constructs.
 using SequencePtr = std::shared_ptr<Sequence>;
 
+class EventTree;  // Manages the order assignment to functional events.
+
 /// Representation of functional events in event trees.
 class FunctionalEvent : public Element {
+  friend class EventTree;
+
  public:
   using Element::Element;
+
+  /// @returns The order of the functional event in the event tree.
+  /// @returns 0 if no order has been assigned.
+  int order() const { return order_; }
+
+ private:
+  /// Sets the functional event order.
+  void order(int order) { order_ = order; }
+
+  int order_ = 0;  ///< The order of the functional event.
 };
 
 /// Functional events are defined in and unique to event trees.