enable adding midi ins and outs

input/src/command.rs

@@ -16,3 +16,13 @@ pub trait Command<S>: Send + Sync + Sized {
         Ok(self.execute(state)?.map(wrap))
     }
 }
+impl<S, T: Command<S>> Command<S> for Option<T> {
+    fn execute (self, _: &mut S) -> Perhaps<Self> {
+        Ok(None)
+    }
+    fn delegate <U> (self, _: &mut S, _: impl Fn(Self)->U) -> Perhaps<U>
+        where Self: Sized
+    {
+        Ok(None)
+    }
+}