Only place real "Ultimate Goal" elements at the bottom of the graph

"Ultimate Goal" elements are only counted as such if they fulfil the
following requirements:

- They have the "Ultimate Goal" issue label
- They are not depended on by any other elements

techtree-manager/src/tree.rs

@@ -199,12 +199,23 @@ impl Tree {
     }
 
     pub fn iter_ultimate_elements<'a>(&'a self) -> impl Iterator<Item = ElementIndex> + 'a {
-        self.graph.node_indices().filter(|index| {
-            // If there are no incoming edges, then this is an ultimate element!
         self.graph
+            .node_indices()
+            .filter(|index| {
+                self.graph.node_weight(*index).unwrap().status == ElementStatus::UltimateGoal
+            })
+            .filter(|index| {
+                // Ultimate goal elements must not have any incoming dependency edges. Warn and
+                // ignore it, if one does.
+                let has_no_incoming = self.graph
                     .neighbors_directed(*index, petgraph::Direction::Incoming)
                     .next()
-                .is_none()
+                    .is_none();
+                if !has_no_incoming {
+                    let el = self.graph.node_weight(*index).unwrap();
+                    log::warn!("Element #{} is marked \"Ultimate Goal\" but is depended on by others? Ignoring...", el.issue_number);
+                }
+                has_no_incoming
             })
     }