diff --git a/nested/src/lib.rs b/nested/src/lib.rs
index 604bb3a..83d3384 100644
--- a/nested/src/lib.rs
+++ b/nested/src/lib.rs
@@ -18,6 +18,7 @@ pub mod vec;
 
 // editors
 pub mod product;
+pub mod sum;
 pub mod list;
 pub mod tree_nav;
 pub mod diagnostics;
diff --git a/nested/src/sum/editor.rs b/nested/src/sum/editor.rs
new file mode 100644
index 0000000..bc590dc
--- /dev/null
+++ b/nested/src/sum/editor.rs
@@ -0,0 +1,101 @@
+use {
+    crate::{
+        core::{ViewPort, OuterViewPort, TypeLadder, Context},
+        terminal::{
+            TerminalEditor, TerminalEditorResult,
+            TerminalEvent, TerminalView
+        },
+        vec::{VecBuffer, MutableVecAccess},
+        index::{buffer::{IndexBuffer, MutableIndexAccess}, IndexView},
+        list::ListCursorMode,
+        product::{segment::ProductEditorSegment},
+        sequence::{SequenceView},
+        make_editor::make_editor,
+
+        tree_nav::{TreeNav, TreeCursor, TerminalTreeEditor, TreeNavResult},
+        diagnostics::{Diagnostics, Message},
+        terminal::{TerminalStyle}
+    },
+    cgmath::{Vector2, Point2},
+    std::sync::{Arc, RwLock},
+    termion::event::{Event, Key},
+    std::ops::{Deref, DerefMut}
+};
+
+pub struct SumEditor {
+    cur: usize,
+    editors: Vec< Arc<RwLock<dyn TerminalTreeEditor + Send + Sync>> >,
+
+    port: ViewPort< dyn TerminalView >,
+    diag_port: OuterViewPort< dyn SequenceView<Item = Message> >
+}
+
+impl SumEditor {
+    pub fn new(
+        editors: Vec< Arc<RwLock<dyn TerminalTreeEditor + Send + Sync>> >
+    ) -> Self {
+        let port = ViewPort::new();
+        let mut diag_buf = VecBuffer::new();
+
+        SumEditor {
+            cur: 0,
+            editors,
+            port,
+            diag_port: diag_buf.get_port().to_sequence()
+        }
+    }
+    
+    pub fn select(&mut self, idx: usize) {
+        self.cur = idx;
+        let tv = self.editors[ self.cur ].read().unwrap().get_term_view();
+        tv.add_observer( self.port.get_cast() );
+        self.port.add_update_hook( Arc::new(tv.0.clone()) );
+        self.port.set_view( Some(tv.get_view_arc()) );
+    }
+}
+
+impl TreeNav for SumEditor {
+    fn get_cursor(&self) -> TreeCursor {
+        self.editors[ self.cur ].write().unwrap().get_cursor()
+    }
+
+    fn get_cursor_warp(&self) -> TreeCursor {
+        self.editors[ self.cur ].write().unwrap().get_cursor_warp()
+    }
+
+    fn goby(&mut self, direction: Vector2<isize>) -> TreeNavResult {
+        self.editors[ self.cur ].write().unwrap().goby( direction )
+    }
+
+    fn goto(&mut self, new_cursor: TreeCursor) -> TreeNavResult {
+        self.editors[ self.cur ].write().unwrap().goto( new_cursor )
+    }
+}
+
+impl TerminalEditor for SumEditor {
+    fn get_term_view(&self) -> OuterViewPort<dyn TerminalView> {
+        self.port.outer()
+    }
+
+    fn handle_terminal_event(&mut self, event: &TerminalEvent) -> TerminalEditorResult {
+        match event {
+            TerminalEvent::Input( termion::event::Event::Key(Key::Ctrl('x')) ) => {
+                self.select( (self.cur + 1) % self.editors.len() );
+                TerminalEditorResult::Continue
+            },
+            event => {
+                self.editors[ self.cur ].write().unwrap().handle_terminal_event( event )
+            }
+        }
+    }
+}
+
+impl Diagnostics for SumEditor {
+    fn get_msg_port(&self) -> OuterViewPort<dyn SequenceView<Item = Message>> {
+        self.diag_port.clone()
+    }
+}
+
+impl TerminalTreeEditor for SumEditor {}
+
+
diff --git a/nested/src/sum/mod.rs b/nested/src/sum/mod.rs
new file mode 100644
index 0000000..7b4d700
--- /dev/null
+++ b/nested/src/sum/mod.rs
@@ -0,0 +1,4 @@
+
+pub mod editor;
+pub use editor::SumEditor;
+