diff --git a/chain/src/txhashset/txhashset.rs b/chain/src/txhashset/txhashset.rs
index 698239b55..7c38bda56 100644
--- a/chain/src/txhashset/txhashset.rs
+++ b/chain/src/txhashset/txhashset.rs
@@ -69,18 +69,12 @@ impl HashOnlyMMRHandle {
 	}
 }
 
-struct PMMRHandle<T>
-where
-	T: PMMRable,
-{
+struct PMMRHandle<T: PMMRable> {
 	backend: PMMRBackend<T>,
 	last_pos: u64,
 }
 
-impl<T> PMMRHandle<T>
-where
-	T: PMMRable + ::std::fmt::Debug,
-{
+impl<T: PMMRable> PMMRHandle<T> {
 	fn new(
 		root_dir: &str,
 		sub_dir: &str,
diff --git a/core/src/core/block.rs b/core/src/core/block.rs
index b7988673c..769c35366 100644
--- a/core/src/core/block.rs
+++ b/core/src/core/block.rs
@@ -35,7 +35,7 @@ use core::{
 use global;
 use keychain::{self, BlindingFactor};
 use pow::{Difficulty, Proof, ProofOfWork};
-use ser::{self, PMMRable, Readable, Reader, Writeable, Writer};
+use ser::{self, HashOnlyPMMRable, Readable, Reader, Writeable, Writer};
 use util::{secp, static_secp_instance};
 
 /// Errors thrown by Block validation
@@ -147,11 +147,9 @@ fn fixed_size_of_serialized_header(_version: u16) -> usize {
 	size += mem::size_of::<u16>(); // version
 	size += mem::size_of::<u64>(); // height
 	size += mem::size_of::<i64>(); // timestamp
-	// prev_hash, prev_root, output_root, range_proof_root, kernel_root
-	size += 5 * mem::size_of::<Hash>();
+	size += 5 * mem::size_of::<Hash>(); // prev_hash, prev_root, output_root, range_proof_root, kernel_root
 	size += mem::size_of::<BlindingFactor>(); // total_kernel_offset
-	// output_mmr_size, kernel_mmr_size
-	size += 2 * mem::size_of::<u64>();
+	size += 2 * mem::size_of::<u64>(); // output_mmr_size, kernel_mmr_size
 	size += mem::size_of::<Difficulty>(); // total_difficulty
 	size += mem::size_of::<u32>(); // secondary_scaling
 	size += mem::size_of::<u64>(); // nonce
@@ -190,13 +188,7 @@ impl Default for BlockHeader {
 	}
 }
 
-/// Block header hashes are maintained in the header MMR
-/// but we store the data itself in the db.
-impl PMMRable for BlockHeader {
-	fn len() -> usize {
-		0
-	}
-}
+impl HashOnlyPMMRable for BlockHeader {}
 
 /// Serialization of a block header
 impl Writeable for BlockHeader {
diff --git a/core/src/core/hash.rs b/core/src/core/hash.rs
index 834747ae1..c52e34208 100644
--- a/core/src/core/hash.rs
+++ b/core/src/core/hash.rs
@@ -26,7 +26,7 @@ use std::{fmt, ops};
 use blake2::blake2b::Blake2b;
 
 use consensus;
-use ser::{self, AsFixedBytes, Error, Readable, Reader, Writeable, Writer};
+use ser::{self, AsFixedBytes, Error, FixedLength, Readable, Reader, Writeable, Writer};
 use util;
 
 /// A hash consisting of all zeroes, used as a sentinel. No known preimage.
@@ -52,15 +52,17 @@ impl fmt::Display for Hash {
 	}
 }
 
-impl Hash {
+impl FixedLength for Hash {
 	/// Size of a hash in bytes.
-	pub const SIZE: usize = 32;
+	const LEN: usize = 32;
+}
 
+impl Hash {
 	/// Builds a Hash from a byte vector. If the vector is too short, it will be
 	/// completed by zeroes. If it's too long, it will be truncated.
 	pub fn from_vec(v: &[u8]) -> Hash {
-		let mut h = [0; Hash::SIZE];
-		let copy_size = min(v.len(), Hash::SIZE);
+		let mut h = [0; Hash::LEN];
+		let copy_size = min(v.len(), Hash::LEN);
 		h[..copy_size].copy_from_slice(&v[..copy_size]);
 		Hash(h)
 	}
diff --git a/core/src/core/pmmr/backend.rs b/core/src/core/pmmr/backend.rs
index b0ca5cdd9..69a098e62 100644
--- a/core/src/core/pmmr/backend.rs
+++ b/core/src/core/pmmr/backend.rs
@@ -34,10 +34,7 @@ pub trait HashOnlyBackend {
 /// The PMMR itself does not need the Backend to be accurate on the existence
 /// of an element (i.e. remove could be a no-op) but layers above can
 /// depend on an accurate Backend to check existence.
-pub trait Backend<T>
-where
-	T: PMMRable,
-{
+pub trait Backend<T: PMMRable> {
 	/// Append the provided Hashes to the backend storage, and optionally an
 	/// associated data element to flatfile storage (for leaf nodes only). The
 	/// position of the first element of the Vec in the MMR is provided to
diff --git a/core/src/core/pmmr/db_pmmr.rs b/core/src/core/pmmr/db_pmmr.rs
index e10f26953..a4a89bc2e 100644
--- a/core/src/core/pmmr/db_pmmr.rs
+++ b/core/src/core/pmmr/db_pmmr.rs
@@ -18,12 +18,12 @@ use std::marker;
 
 use core::hash::{Hash, ZERO_HASH};
 use core::pmmr::{bintree_postorder_height, is_leaf, peak_map_height, peaks, HashOnlyBackend};
-use ser::{PMMRIndexHashable, PMMRable};
+use ser::{HashOnlyPMMRable, PMMRIndexHashable};
 
 /// Database backed MMR.
 pub struct DBPMMR<'a, T, B>
 where
-	T: PMMRable,
+	T: HashOnlyPMMRable,
 	B: 'a + HashOnlyBackend,
 {
 	/// The last position in the PMMR
@@ -36,7 +36,7 @@ where
 
 impl<'a, T, B> DBPMMR<'a, T, B>
 where
-	T: PMMRable + ::std::fmt::Debug,
+	T: HashOnlyPMMRable,
 	B: 'a + HashOnlyBackend,
 {
 	/// Build a new db backed MMR.
diff --git a/core/src/core/pmmr/pmmr.rs b/core/src/core/pmmr/pmmr.rs
index db55d2829..0850bc517 100644
--- a/core/src/core/pmmr/pmmr.rs
+++ b/core/src/core/pmmr/pmmr.rs
@@ -47,7 +47,7 @@ where
 
 impl<'a, T, B> PMMR<'a, T, B>
 where
-	T: PMMRable + ::std::fmt::Debug,
+	T: PMMRable,
 	B: 'a + Backend<T>,
 {
 	/// Build a new prunable Merkle Mountain Range using the provided backend.
diff --git a/core/src/core/pmmr/readonly_pmmr.rs b/core/src/core/pmmr/readonly_pmmr.rs
index c1da43369..8fdeaf346 100644
--- a/core/src/core/pmmr/readonly_pmmr.rs
+++ b/core/src/core/pmmr/readonly_pmmr.rs
@@ -35,7 +35,7 @@ where
 
 impl<'a, T, B> ReadonlyPMMR<'a, T, B>
 where
-	T: PMMRable + ::std::fmt::Debug,
+	T: PMMRable,
 	B: 'a + Backend<T>,
 {
 	/// Build a new readonly PMMR.
diff --git a/core/src/core/pmmr/rewindable_pmmr.rs b/core/src/core/pmmr/rewindable_pmmr.rs
index d220a8c75..2f8854383 100644
--- a/core/src/core/pmmr/rewindable_pmmr.rs
+++ b/core/src/core/pmmr/rewindable_pmmr.rs
@@ -37,7 +37,7 @@ where
 
 impl<'a, T, B> RewindablePMMR<'a, T, B>
 where
-	T: PMMRable + ::std::fmt::Debug,
+	T: PMMRable,
 	B: 'a + Backend<T>,
 {
 	/// Build a new readonly PMMR.
diff --git a/core/src/core/transaction.rs b/core/src/core/transaction.rs
index 1dc43711f..c94df17df 100644
--- a/core/src/core/transaction.rs
+++ b/core/src/core/transaction.rs
@@ -26,7 +26,7 @@ use core::hash::Hashed;
 use core::verifier_cache::VerifierCache;
 use core::{committed, Committed};
 use keychain::{self, BlindingFactor};
-use ser::{self, read_multi, PMMRable, Readable, Reader, Writeable, Writer};
+use ser::{self, read_multi, FixedLength, PMMRable, Readable, Reader, Writeable, Writer};
 use util;
 use util::secp::pedersen::{Commitment, RangeProof};
 use util::secp::{self, Message, Signature};
@@ -240,13 +240,14 @@ impl TxKernel {
 	}
 }
 
-impl PMMRable for TxKernel {
-	fn len() -> usize {
-		17 + // features plus fee and lock_height
-			secp::constants::PEDERSEN_COMMITMENT_SIZE + secp::constants::AGG_SIGNATURE_SIZE
-	}
+impl FixedLength for TxKernel {
+	const LEN: usize = 17 // features plus fee and lock_height
+		+ secp::constants::PEDERSEN_COMMITMENT_SIZE
+		+ secp::constants::AGG_SIGNATURE_SIZE;
 }
 
+impl PMMRable for TxKernel {}
+
 /// TransactionBody is a common abstraction for transaction and block
 #[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct TransactionBody {
@@ -1172,13 +1173,12 @@ impl OutputIdentifier {
 	}
 }
 
-/// Ensure this is implemented to centralize hashing with indexes
-impl PMMRable for OutputIdentifier {
-	fn len() -> usize {
-		1 + secp::constants::PEDERSEN_COMMITMENT_SIZE
-	}
+impl FixedLength for OutputIdentifier {
+	const LEN: usize = 1 + secp::constants::PEDERSEN_COMMITMENT_SIZE;
 }
 
+impl PMMRable for OutputIdentifier {}
+
 impl Writeable for OutputIdentifier {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
 		writer.write_u8(self.features.bits())?;
diff --git a/core/src/ser.rs b/core/src/ser.rs
index 73495834e..63de71cdb 100644
--- a/core/src/ser.rs
+++ b/core/src/ser.rs
@@ -23,6 +23,7 @@ use byteorder::{BigEndian, ByteOrder, ReadBytesExt};
 use consensus;
 use core::hash::{Hash, Hashed};
 use keychain::{BlindingFactor, Identifier, IDENTIFIER_SIZE};
+use std::fmt::Debug;
 use std::io::{self, Read, Write};
 use std::{cmp, error, fmt, mem};
 use util::secp::constants::{
@@ -371,12 +372,12 @@ impl Readable for RangeProof {
 	}
 }
 
-impl PMMRable for RangeProof {
-	fn len() -> usize {
-		MAX_PROOF_SIZE + 8
-	}
+impl FixedLength for RangeProof {
+	const LEN: usize = MAX_PROOF_SIZE + 8;
 }
 
+impl PMMRable for RangeProof {}
+
 impl Readable for Signature {
 	fn read(reader: &mut Reader) -> Result<Signature, Error> {
 		let a = reader.read_fixed_bytes(AGG_SIGNATURE_SIZE)?;
@@ -535,31 +536,30 @@ impl Writeable for [u8; 4] {
 	}
 }
 
-/// Trait for types that can serialize and report their size
-pub trait PMMRable: Readable + Writeable + Clone {
-	/// Length in bytes
-	fn len() -> usize;
+/// Trait for types that serialize to a known fixed length.
+pub trait FixedLength {
+	/// The length in bytes
+	const LEN: usize;
 }
 
+/// Trait for types that can be added to a "hash only" PMMR (block headers for example).
+pub trait HashOnlyPMMRable: Writeable + Clone + Debug {}
+
+/// Trait for types that can be added to a PMMR.
+pub trait PMMRable: FixedLength + Readable + Writeable + Clone + Debug {}
+
 /// Generic trait to ensure PMMR elements can be hashed with an index
 pub trait PMMRIndexHashable {
 	/// Hash with a given index
 	fn hash_with_index(&self, index: u64) -> Hash;
 }
 
-impl<T: PMMRable> PMMRIndexHashable for T {
+impl<T: Writeable> PMMRIndexHashable for T {
 	fn hash_with_index(&self, index: u64) -> Hash {
 		(index, self).hash()
 	}
 }
 
-// Convenient way to hash two existing hashes together with an index.
-impl PMMRIndexHashable for (Hash, Hash) {
-	fn hash_with_index(&self, index: u64) -> Hash {
-		(index, &self.0, &self.1).hash()
-	}
-}
-
 /// Useful marker trait on types that can be sized byte slices
 pub trait AsFixedBytes: Sized + AsRef<[u8]> {
 	/// The length in bytes
diff --git a/core/tests/vec_backend/mod.rs b/core/tests/vec_backend/mod.rs
index c51de08e8..06a36f833 100644
--- a/core/tests/vec_backend/mod.rs
+++ b/core/tests/vec_backend/mod.rs
@@ -20,17 +20,17 @@ use core::core::hash::Hash;
 use core::core::pmmr::{self, Backend};
 use core::core::BlockHeader;
 use core::ser;
-use core::ser::{PMMRable, Readable, Reader, Writeable, Writer};
+use core::ser::{FixedLength, PMMRable, Readable, Reader, Writeable, Writer};
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub struct TestElem(pub [u32; 4]);
 
-impl PMMRable for TestElem {
-	fn len() -> usize {
-		16
-	}
+impl FixedLength for TestElem {
+	const LEN: usize = 16;
 }
 
+impl PMMRable for TestElem {}
+
 impl Writeable for TestElem {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
 		try!(writer.write_u32(self.0[0]));
@@ -54,10 +54,7 @@ impl Readable for TestElem {
 /// Simple MMR backend implementation based on a Vector. Pruning does not
 /// compact the Vec itself.
 #[derive(Clone, Debug)]
-pub struct VecBackend<T>
-where
-	T: PMMRable,
-{
+pub struct VecBackend<T: PMMRable> {
 	/// Backend elements
 	pub data: Vec<T>,
 	pub hashes: Vec<Hash>,
@@ -65,10 +62,7 @@ where
 	pub remove_list: Vec<u64>,
 }
 
-impl<T> Backend<T> for VecBackend<T>
-where
-	T: PMMRable,
-{
+impl<T: PMMRable> Backend<T> for VecBackend<T> {
 	fn append(&mut self, data: T, hashes: Vec<Hash>) -> Result<(), String> {
 		self.data.push(data);
 		self.hashes.append(&mut hashes.clone());
@@ -125,10 +119,7 @@ where
 	fn dump_stats(&self) {}
 }
 
-impl<T> VecBackend<T>
-where
-	T: PMMRable,
-{
+impl<T: PMMRable> VecBackend<T> {
 	/// Instantiates a new VecBackend<T>
 	pub fn new() -> VecBackend<T> {
 		VecBackend {
diff --git a/store/src/pmmr.rs b/store/src/pmmr.rs
index 31a2772e1..b7678b91b 100644
--- a/store/src/pmmr.rs
+++ b/store/src/pmmr.rs
@@ -20,7 +20,7 @@ use croaring::Bitmap;
 use core::core::hash::{Hash, Hashed};
 use core::core::pmmr::{self, family, Backend, HashOnlyBackend};
 use core::core::BlockHeader;
-use core::ser::{self, PMMRable};
+use core::ser::{self, FixedLength, PMMRable};
 use leaf_set::LeafSet;
 use prune_list::PruneList;
 use types::{prune_noop, AppendOnlyFile, HashFile};
@@ -49,10 +49,7 @@ pub const PMMR_FILES: [&str; 4] = [
 /// * A leaf_set tracks unpruned (unremoved) leaf positions in the MMR..
 /// * A prune_list tracks the positions of pruned (and compacted) roots in the
 /// MMR.
-pub struct PMMRBackend<T>
-where
-	T: PMMRable,
-{
+pub struct PMMRBackend<T: PMMRable> {
 	data_dir: String,
 	prunable: bool,
 	hash_file: AppendOnlyFile,
@@ -62,16 +59,13 @@ where
 	_marker: marker::PhantomData<T>,
 }
 
-impl<T> Backend<T> for PMMRBackend<T>
-where
-	T: PMMRable + ::std::fmt::Debug,
-{
+impl<T: PMMRable> Backend<T> for PMMRBackend<T> {
 	/// Append the provided data and hashes to the backend storage.
 	/// Add the new leaf pos to our leaf_set if this is a prunable MMR.
 	#[allow(unused_variables)]
 	fn append(&mut self, data: T, hashes: Vec<Hash>) -> Result<(), String> {
 		if self.prunable {
-			let record_len = Hash::SIZE as u64;
+			let record_len = Hash::LEN as u64;
 			let shift = self.prune_list.get_total_shift();
 			let position = (self.hash_file.size_unsync() / record_len) + shift + 1;
 			self.leaf_set.add(position);
@@ -95,7 +89,7 @@ where
 		let pos = position - 1;
 
 		// Must be on disk, doing a read at the correct position
-		let hash_record_len = Hash::SIZE;
+		let hash_record_len = Hash::LEN;
 		let file_offset = ((pos - shift) as usize) * hash_record_len;
 		let data = self.hash_file.read(file_offset, hash_record_len);
 		match ser::deserialize(&mut &data[..]) {
@@ -118,7 +112,7 @@ where
 		let pos = pmmr::n_leaves(position) - 1;
 
 		// Must be on disk, doing a read at the correct position
-		let record_len = T::len();
+		let record_len = T::LEN;
 		let file_offset = ((pos - shift) as usize) * record_len;
 		let data = self.data_file.read(file_offset, record_len);
 		match ser::deserialize(&mut &data[..]) {
@@ -164,14 +158,14 @@ where
 
 		// Rewind the hash file accounting for pruned/compacted pos
 		let shift = self.prune_list.get_shift(position);
-		let record_len = Hash::SIZE as u64;
+		let record_len = Hash::LEN as u64;
 		let file_pos = (position - shift) * record_len;
 		self.hash_file.rewind(file_pos);
 
 		// Rewind the data file accounting for pruned/compacted pos
 		let leaf_shift = self.prune_list.get_leaf_shift(position);
 		let flatfile_pos = pmmr::n_leaves(position);
-		let record_len = T::len() as u64;
+		let record_len = T::LEN as u64;
 		let file_pos = (flatfile_pos - leaf_shift) * record_len;
 		self.data_file.rewind(file_pos);
 
@@ -209,10 +203,7 @@ where
 	}
 }
 
-impl<T> PMMRBackend<T>
-where
-	T: PMMRable + ::std::fmt::Debug,
-{
+impl<T: PMMRable> PMMRBackend<T> {
 	/// Instantiates a new PMMR backend.
 	/// Use the provided dir to store its files.
 	pub fn new(
@@ -263,7 +254,7 @@ where
 	pub fn unpruned_size(&self) -> io::Result<u64> {
 		let total_shift = self.prune_list.get_total_shift();
 
-		let record_len = Hash::SIZE as u64;
+		let record_len = Hash::LEN as u64;
 		let sz = self.hash_file.size()?;
 		Ok(sz / record_len + total_shift)
 	}
@@ -271,14 +262,14 @@ where
 	/// Number of elements in the underlying stored data. Extremely dependent on
 	/// pruning and compaction.
 	pub fn data_size(&self) -> io::Result<u64> {
-		let record_len = T::len() as u64;
+		let record_len = T::LEN as u64;
 		self.data_file.size().map(|sz| sz / record_len)
 	}
 
 	/// Size of the underlying hashed data. Extremely dependent on pruning
 	/// and compaction.
 	pub fn hash_size(&self) -> io::Result<u64> {
-		self.hash_file.size().map(|sz| sz / Hash::SIZE as u64)
+		self.hash_file.size().map(|sz| sz / Hash::LEN as u64)
 	}
 
 	/// Syncs all files to disk. A call to sync is required to ensure all the
@@ -348,7 +339,7 @@ where
 
 		// 1. Save compact copy of the hash file, skipping removed data.
 		{
-			let record_len = Hash::SIZE as u64;
+			let record_len = Hash::LEN as u64;
 
 			let off_to_rm = map_vec!(pos_to_rm, |pos| {
 				let shift = self.prune_list.get_shift(pos.into());
@@ -365,7 +356,7 @@ where
 
 		// 2. Save compact copy of the data file, skipping removed leaves.
 		{
-			let record_len = T::len() as u64;
+			let record_len = T::LEN as u64;
 
 			let leaf_pos_to_rm = pos_to_rm
 				.iter()
@@ -488,7 +479,7 @@ impl HashOnlyMMRBackend {
 	/// The unpruned size of this MMR backend.
 	pub fn unpruned_size(&self) -> io::Result<u64> {
 		let sz = self.hash_file.size()?;
-		Ok(sz / Hash::SIZE as u64)
+		Ok(sz / Hash::LEN as u64)
 	}
 
 	/// Discard any pending changes to this MMR backend.
diff --git a/store/src/types.rs b/store/src/types.rs
index 985b0dcea..ea21d4dd1 100644
--- a/store/src/types.rs
+++ b/store/src/types.rs
@@ -26,7 +26,7 @@ use libc::{ftruncate as ftruncate64, off_t as off64_t};
 use libc::{ftruncate64, off64_t};
 
 use core::core::hash::Hash;
-use core::ser;
+use core::ser::{self, FixedLength};
 
 /// A no-op function for doing nothing with some pruned data.
 pub fn prune_noop(_pruned_data: &[u8]) {}
@@ -58,8 +58,8 @@ impl HashFile {
 		let pos = position - 1;
 
 		// Must be on disk, doing a read at the correct position
-		let file_offset = (pos as usize) * Hash::SIZE;
-		let data = self.file.read(file_offset, Hash::SIZE);
+		let file_offset = (pos as usize) * Hash::LEN;
+		let data = self.file.read(file_offset, Hash::LEN);
 		match ser::deserialize(&mut &data[..]) {
 			Ok(h) => Some(h),
 			Err(e) => {
@@ -74,7 +74,7 @@ impl HashFile {
 
 	/// Rewind the backend file to the specified position.
 	pub fn rewind(&mut self, position: u64) -> io::Result<()> {
-		self.file.rewind(position * Hash::SIZE as u64);
+		self.file.rewind(position * Hash::LEN as u64);
 		Ok(())
 	}
 
diff --git a/store/tests/pmmr.rs b/store/tests/pmmr.rs
index f4ef01e55..966ff2657 100644
--- a/store/tests/pmmr.rs
+++ b/store/tests/pmmr.rs
@@ -24,7 +24,9 @@ use chrono::prelude::Utc;
 use croaring::Bitmap;
 
 use core::core::pmmr::{Backend, PMMR};
-use core::ser::{Error, PMMRIndexHashable, PMMRable, Readable, Reader, Writeable, Writer};
+use core::ser::{
+	Error, FixedLength, PMMRIndexHashable, PMMRable, Readable, Reader, Writeable, Writer,
+};
 use store::types::prune_noop;
 
 #[test]
@@ -752,12 +754,12 @@ fn load(pos: u64, elems: &[TestElem], backend: &mut store::pmmr::PMMRBackend<Tes
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 struct TestElem(u32);
 
-impl PMMRable for TestElem {
-	fn len() -> usize {
-		4
-	}
+impl FixedLength for TestElem {
+	const LEN: usize = 4;
 }
 
+impl PMMRable for TestElem {}
+
 impl Writeable for TestElem {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), Error> {
 		writer.write_u32(self.0)