Added invariants to the Sequence to hunt down a bug in the tests but I'll leave it there for future testing.

animate2.hpp

@@ -11,6 +11,7 @@
 #include "ease2.hpp"
 #include <fmt/core.h>
 #include "json_mods.hpp"
+#include <source_location>
 
 namespace animate2 {
 
@@ -44,11 +45,13 @@ namespace animate2 {
     std::vector<int> durations{}; // in ticks
     size_t current{0};
     int loop_count{0};
-    size_t frame_count{0};
+    size_t frame_count{frames.size()};
     Timer timer{};
     int subframe{0};
     float easing_duration{0.0f};
     float easing_position{0.0f};
+
+    void INVARIANT(const std::source_location location = std::source_location::current());
   };
 
   struct Transform {