diff --git a/2019/24.cpp b/2019/24.cpp
index 4be6e90..ab8c3c7 100644
--- a/2019/24.cpp
+++ b/2019/24.cpp
@@ -33,7 +33,7 @@ struct Map {
   }
 };
 
-auto FindBugs(Map const& map) {
+auto FindBugs(Map const& map) -> std::vector<Coord> {
   std::vector<Coord> result;
 
   std::int64_t w = map.rows[0].size();
@@ -82,12 +82,12 @@ struct Neighbor2 {
   }
 };
 
-template<typename F, typename C>
+template<std::totally_ordered C, std::invocable<C, void(C const&)> F>
 auto Step(std::vector<C> const& bugs, F with_neighbors = F{})
 {
   std::map<C, int> adjacent;
   for (auto const& x : bugs) {
-    with_neighbors(x, [&adjacent](C n) -> void { adjacent[n]++; });
+    with_neighbors(x, [&adjacent](C const& n) -> void { adjacent[n]++; });
   }
 
   std::vector<C> result;
@@ -120,18 +120,18 @@ auto Part1(std::vector<Coord> const& bugs) -> std::uint32_t {
   std::set<std::uint32_t> seen;
   std::uint32_t bio;
   while (seen.insert(bio = Bio(cursor)).second) {
-    cursor = Step<Neighbor1>(cursor);
+    cursor = Step<Coord, Neighbor1>(cursor);
   }
   return bio;
 }
 
 auto Part2(std::vector<Coord> const& bugs) {
-  std::vector<std::pair<Coord,std::int64_t>> bugs2;
+  std::vector<Neighbor2::C3> bugs2;
   for (auto const& [x,y] : bugs) {
     bugs2.push_back({{x-2,y-2},0});
   }
   for (int i = 0; i < 200; i++) {
-    bugs2 = Step<Neighbor2>(bugs2);
+    bugs2 = Step<Neighbor2::C3, Neighbor2>(bugs2);
   }
   return bugs2.size();
 }