singleton2sequence: value only at first element

fixes chain with singleton tosequence filter

nested/src/list/editor.rs

@@ -66,18 +66,8 @@ impl ListEditor {
             addr_port: VecBuffer::<OuterViewPort<dyn SequenceView<Item = isize>>>::with_data(
                 vec![
                     cursor.get_port()
-                        .map(
-                            |x| {
-                                // todo implement this with filter_map
-                                let mut b = VecBuffer::new();
-                                if let Some(i) = x.idx {
-                                    b.push(i);
-                                }
-                                b.get_port().to_sequence()
-                            }
-                        )
                         .to_sequence()
-                        .flatten(),
+                        .filter_map(|cur| cur.idx),
                     cursor.get_port()
                         .map({
                             let data = data.clone();

nested/src/singleton/to_sequence.rs

@@ -51,8 +51,12 @@ where
 {
     type Item = SrcView::Item;
 
-    fn get(&self, _idx: &usize) -> Option<Self::Item> {
-        Some(self.src_view.as_ref()?.get())
+    fn get(&self, idx: &usize) -> Option<Self::Item> {
+        if *idx == 0 {
+            Some(self.src_view.as_ref()?.get())
+        } else {
+            None
+        }
     }
 
     fn len(&self) -> Option<usize> {