warn about time travel

src/physics.rs

@@ -1655,6 +1655,9 @@ fn atomic_input_instruction(state:&mut PhysicsState,data:&PhysicsData,ins:TimedI
 			|PhysicsInstruction::Internal(PhysicsInternalInstruction::ReachWalkTargetVelocity)=>(),
 			_=>println!("{}|{:?}",ins.time,ins.instruction),
 		}
+		if ins.time<state.time{
+			println!("@@@@ Time travel warning! {:?}",ins);
+		}
 		state.time=ins.time;
 		match ins.instruction{
 			PhysicsInstruction::Internal(instruction)=>atomic_internal_instruction(state,data,TimedInstruction{time:ins.time,instruction}),