clean up a bit

src/core/sense.rs

@@ -264,13 +264,9 @@ impl<E: Event<Ctx, Data = <Senses as Event<Ctx>>::Data> + Into<Senses>, Ctx: 'st
         });
     }
 
-    fn run<'a>(
-        &mut self,
-        id: &Id,
-        event: E,
-    ) -> Option<impl Fn(&mut Ctx, E::Data) + use<'a, E, Ctx>> {
+    fn run<'a>(&self, id: &Id, event: E) -> Option<impl Fn(&mut Ctx, E::Data) + use<'a, E, Ctx>> {
         let senses = event.into();
-        if let Some(group) = self.map.get_mut(id) {
+        if let Some(group) = self.map.get(id) {
             let fs: Vec<_> = group
                 .sensors
                 .iter()