diff --git a/src/hashdb.rs b/src/hashdb.rs
index 561c7843d..dd0973bb3 100644
--- a/src/hashdb.rs
+++ b/src/hashdb.rs
@@ -1,4 +1,5 @@
 use hash::*;
+use bytes::*;
 
 pub trait HashDB {
 	/// Look up a given hash into the bytes that hash to it, returning None if the 
@@ -56,6 +57,9 @@ pub trait HashDB {
 	/// ```
 	fn insert(&mut self, value: &[u8]) -> H256;
 
+	/// Like `insert()` , except you provide the key and the data is all moved.
+	fn emplace(&mut self, key: H256, value: Bytes);
+
 	/// Remove a datum previously inserted. Insertions can be "owed" such that the same number of `insert()`s may
 	/// happen without the data being eventually being inserted into the DB.
 	///
diff --git a/src/memorydb.rs b/src/memorydb.rs
index 4ec02bd5e..14fae12d0 100644
--- a/src/memorydb.rs
+++ b/src/memorydb.rs
@@ -37,6 +37,12 @@ use std::collections::HashMap;
 ///   m.kill(&k);
 ///   assert!(!m.exists(&k));
 ///
+///   m.kill(&k);
+///   assert!(!m.exists(&k));
+///
+///   m.insert(d);
+///   assert!(!m.exists(&k));
+
 ///   m.insert(d);
 ///   assert!(m.exists(&k));
 ///   assert_eq!(m.lookup(&k).unwrap(), d);
@@ -130,9 +136,9 @@ impl HashDB for MemoryDB {
 	fn insert(&mut self, value: &[u8]) -> H256 {
 		let key = value.sha3();
 		if match self.data.get_mut(&key) {
-			Some(&mut (ref mut old_value, ref mut rc @ 0)) => {
+			Some(&mut (ref mut old_value, ref mut rc @ -0x80000000i32 ... 0)) => {
 				*old_value = From::from(value.bytes());
-				*rc = 1;
+				*rc += 1;
 				false
 			},
 			Some(&mut (_, ref mut x)) => { *x += 1; false } ,
@@ -143,6 +149,20 @@ impl HashDB for MemoryDB {
 		key
 	}
 
+	fn emplace(&mut self, key: H256, value: Bytes) {
+		match self.data.get_mut(&key) {
+			Some(&mut (ref mut old_value, ref mut rc @ -0x80000000i32 ... 0)) => {
+				*old_value = value;
+				*rc += 1;
+				return;
+			},
+			Some(&mut (_, ref mut x)) => { *x += 1; return; } ,
+			None => {},
+		}
+		// ... None falls through into...
+		self.data.insert(key, (value, 1));
+	}
+
 	fn kill(&mut self, key: &H256) {
 		if match self.data.get_mut(key) {
 			Some(&mut (_, ref mut x)) => { *x -= 1; false }
diff --git a/src/nibbleslice.rs b/src/nibbleslice.rs
index e49244325..5866ed8f0 100644
--- a/src/nibbleslice.rs
+++ b/src/nibbleslice.rs
@@ -1,5 +1,6 @@
 //! Nibble-orientated view onto byte-slice, allowing nibble-precision offsets.
 use std::cmp::*;
+use bytes::*;
 
 /// Nibble-orientated view onto byte-slice, allowing nibble-precision offsets.
 ///
@@ -69,6 +70,18 @@ impl<'a> NibbleSlice<'a> {
 		}
 		i
 	}
+
+	pub fn encoded(&self, is_leaf: bool) -> Bytes {
+		let l = self.len();
+		let mut r = Bytes::with_capacity(l / 2 + 1);
+		let mut i = l % 2;
+		r.push(if i == 1 {0x10 + self.at(0)} else {0} + if is_leaf {0x20} else {0});
+		while i < l {
+			r.push(self.at(i) * 16 + self.at(i + 1));
+			i += 2;
+		}
+		r
+	}
 }
 
 impl<'a> PartialEq for NibbleSlice<'a> {
@@ -126,6 +139,15 @@ mod tests {
 		}
 	}
 
+	#[test]
+	fn encoded() {
+		let n = NibbleSlice::new(D);
+		assert_eq!(n.encoded(false), &[0x00, 0x01, 0x23, 0x45]);
+		assert_eq!(n.encoded(true), &[0x20, 0x01, 0x23, 0x45]);
+		assert_eq!(n.mid(1).encoded(false), &[0x11, 0x23, 0x45]);
+		assert_eq!(n.mid(1).encoded(true), &[0x31, 0x23, 0x45]);
+	}
+
 	#[test]
 	fn shared() {
 		let n = NibbleSlice::new(D);
diff --git a/src/overlaydb.rs b/src/overlaydb.rs
index b6f2b3eb2..3c35d7255 100644
--- a/src/overlaydb.rs
+++ b/src/overlaydb.rs
@@ -186,6 +186,7 @@ impl HashDB for OverlayDB {
 		}
 	}
 	fn insert(&mut self, value: &[u8]) -> H256 { self.overlay.insert(value) }
+	fn emplace(&mut self, key: H256, value: Bytes) { self.overlay.emplace(key, value); }
 	fn kill(&mut self, key: &H256) { self.overlay.kill(key); }
 }
 
diff --git a/src/trie.rs b/src/trie.rs
index 889c9b5dc..80e753f84 100644
--- a/src/trie.rs
+++ b/src/trie.rs
@@ -1,8 +1,9 @@
 use memorydb::*;
 use hashdb::*;
 use hash::*;
-//use rlp::*;
-//use bytes::*;
+use nibbleslice::*;
+use bytes::*;
+use rlp::*;
 
 pub const NULL_RLP: [u8; 1] = [0x80; 1];
 pub const SHA3_NULL_RLP: H256 = H256( [0x56, 0xe8, 0x1f, 0x17, 0x1b, 0xcc, 0x55, 0xa6, 0xff, 0x83, 0x45, 0xe6, 0x92, 0xc0, 0xf8, 0x6e, 0x5b, 0x48, 0xe0, 0x1b, 0x99, 0x6c, 0xad, 0xc0, 0x01, 0x62, 0x2f, 0xb5, 0xe3, 0x63, 0xb4, 0x21] );
@@ -17,10 +18,10 @@ pub trait Trie {
 	fn is_empty(&self) -> bool { *self.root() == SHA3_NULL_RLP }
 
 	// TODO: consider returning &[u8]...
-	fn contains(key: &[u8]) -> bool;
-	fn at(key: &[u8]) -> Option<&[u8]>;
-	fn insert(key: &[u8], value: &[u8]);
-	fn remove(key: &[u8]);
+	fn contains(&self, key: &[u8]) -> bool;
+	fn at(&self, key: &[u8]) -> Option<&[u8]>;
+	fn insert(&mut self, key: &[u8], value: &[u8]);
+	fn remove(&mut self, key: &[u8]);
 }
 
 pub struct TrieDB {
@@ -28,6 +29,11 @@ pub struct TrieDB {
 	root: H256,
 }
 
+struct Diff {
+	new: Vec<(H256, Bytes)>,
+	old: Vec<H256>,
+}
+
 impl TrieDB {
 	pub fn new<T>(db: T) -> Self where T: HashDB + 'static { TrieDB{ db: Box::new(db), root: H256::new() } }
 
@@ -35,36 +41,85 @@ impl TrieDB {
 
 	pub fn new_memory() -> Self { TrieDB{ db: Box::new(MemoryDB::new()), root: H256::new() } }
 
-	pub fn init(&mut self) { self.insert_root(&NULL_RLP); }
+	pub fn init(&mut self) { self.set_root_rlp(&NULL_RLP); }
 
 	pub fn db(&self) -> &HashDB { self.db.as_ref() }
 
-	fn insert_root(&mut self, root_data: &[u8]) { self.root = self.db.insert(root_data); }
+	fn set_root_rlp(&mut self, root_data: &[u8]) {
+		self.root = self.db.insert(root_data);
+	}
 
+	fn insert(&mut self, key: &NibbleSlice, value: &[u8]) {
+		// determine what the new root is, insert new nodes and remove old as necessary.
+		let mut todo: (Bytes, Diff);
+		{
+			let root_rlp = self.db.lookup(&self.root).unwrap();
+			todo = self.merge(root_rlp, key, value);
+		}
+		self.apply(todo.1);
+		self.set_root_rlp(&todo.0);
+	}
+
+	fn apply(&mut self, diff: Diff) {
+		for d in diff.old.iter() {
+			self.db.kill(&d);
+		}
+		for d in diff.new.into_iter() {
+			self.db.emplace(d.0, d.1);
+		}
+	}
+
+	/// Determine the RLP of the node, assuming we're inserting `partial_key` into the
+	/// node at `old`. This will *not* delete the old mode; it will just return the new RLP
+	/// that includes the new node.
+	///
+	/// The database will be updated so as to make the returned RLP valid through inserting
+	/// and deleting nodes as necessary.
+	fn merge(&self, old: &[u8], partial_key: &NibbleSlice, value: &[u8]) -> (Bytes, Diff) {
+		let o = Rlp::new(old);
+		match (o.type()) {
+			List(17) => {
+				// already have a branch. route and merge.
+			},
+			List(2) => {
+				// already have an extension. either fast_forward, cleve or transmute_to_branch.
+			},
+			Data(0) => compose_extension(partial_key),
+			_ -> panic!("Invalid RLP for node."),
+		}
+	}
+
+	fn compose_extension(partial_key: &NibbleSlice, value: &[u8], is_leaf: bool) -> Bytes {
+		let mut s = RlpStream::new_list(2);
+		s.append(&partial_key.encoded(is_leaf));
+		s.append(&value.to_vec());	// WTF?!?!
+		//s.append(value);	// <-- should be.
+		s.out().unwrap()
+	}
 }
 
 impl Trie for TrieDB {
 	fn root(&self) -> &H256 { &self.root }
 
-	fn contains(_key: &[u8]) -> bool {
+	fn contains(&self, _key: &[u8]) -> bool {
 		unimplemented!();
 	}
 
-	fn at(_key: &[u8]) -> Option<&[u8]> {
+	fn at(&self, _key: &[u8]) -> Option<&[u8]> {
 		unimplemented!();
 	}
 
-	fn insert(_key: &[u8], _value: &[u8]) {
-		unimplemented!();
+	fn insert(&mut self, key: &[u8], value: &[u8]) {
+		(self as &mut TrieDB).insert(&NibbleSlice::new(key), value);
 	}
 
-	fn remove(_key: &[u8]) {
+	fn remove(&mut self, _key: &[u8]) {
 		unimplemented!();
 	}
 }
 
 #[test]
-fn it_works() {
+fn playpen() {
 	use overlaydb::*;
 
 	(&[1, 2, 3]).starts_with(&[1, 2]);
@@ -73,6 +128,5 @@ fn it_works() {
 	t.init();
 	assert_eq!(*t.root(), SHA3_NULL_RLP);
 	assert!(t.is_empty());
-	// TODO: make work:
-	//assert_eq!(t.root(), SHA3_NULL_RLP);
+	t.insert(&[0x01u8, 0x23], &[0x01u8, 0x23]);
 }
\ No newline at end of file