lib.rs

		dest: &T::AccountId,
		value: Self::Balance,
		existence_requirement: ExistenceRequirement,
	) -> DispatchResult {
		if value.is_zero() || transactor == dest { return Ok(()) }

		Self::try_mutate_account(dest, |to_account, _| -> DispatchResult {
			Self::try_mutate_account(transactor, |from_account, _| -> DispatchResult {
				from_account.free = from_account.free.checked_sub(&value)
					.ok_or(Error::<T, I>::InsufficientBalance)?;

				// NOTE: total stake being stored in the same type means that this could never overflow
				// but better to be safe than sorry.
				to_account.free = to_account.free.checked_add(&value).ok_or(Error::<T, I>::Overflow)?;

				let ed = T::ExistentialDeposit::get();
				ensure!(to_account.total() >= ed, Error::<T, I>::ExistentialDeposit);

				Self::ensure_can_withdraw(
					transactor,
					value,
					WithdrawReason::Transfer.into(),
					from_account.free,
				)?;

				let allow_death = existence_requirement == ExistenceRequirement::AllowDeath;
				let allow_death = allow_death && system::Module::<T>::allow_death(transactor);
				ensure!(allow_death || from_account.free >= ed, Error::<T, I>::KeepAlive);

				Ok(())
			})
		})?;

		// Emit transfer event.
		Self::deposit_event(RawEvent::Transfer(transactor.clone(), dest.clone(), value));

		Ok(())
	}

	/// Slash a target account `who`, returning the negative imbalance created and any left over
	/// amount that could not be slashed.
	///
	/// Is a no-op if `value` to be slashed is zero.
	///
	/// NOTE: `slash()` prefers free balance, but assumes that reserve balance can be drawn
	/// from in extreme circumstances. `can_slash()` should be used prior to `slash()` to avoid having
	/// to draw from reserved funds, however we err on the side of punishment if things are inconsistent
	/// or `can_slash` wasn't used appropriately.
	fn slash(
		who: &T::AccountId,
		value: Self::Balance
	) -> (Self::NegativeImbalance, Self::Balance) {
		if value.is_zero() { return (NegativeImbalance::zero(), Zero::zero()) }

		Self::mutate_account(who, |account| {
			let free_slash = cmp::min(account.free, value);
			account.free -= free_slash;

			let remaining_slash = value - free_slash;
			if !remaining_slash.is_zero() {
				let reserved_slash = cmp::min(account.reserved, remaining_slash);
				account.reserved -= reserved_slash;
				(NegativeImbalance::new(free_slash + reserved_slash), remaining_slash - reserved_slash)
			} else {
				(NegativeImbalance::new(value), Zero::zero())
			}
		})
	}

	/// Deposit some `value` into the free balance of an existing target account `who`.
	///
	/// Is a no-op if the `value` to be deposited is zero.
	fn deposit_into_existing(
		who: &T::AccountId,
		value: Self::Balance
	) -> Result<Self::PositiveImbalance, DispatchError> {
		if value.is_zero() { return Ok(PositiveImbalance::zero()) }

		Self::try_mutate_account(who, |account, is_new| -> Result<Self::PositiveImbalance, DispatchError> {
			ensure!(!is_new, Error::<T, I>::DeadAccount);
			account.free = account.free.checked_add(&value).ok_or(Error::<T, I>::Overflow)?;
			Ok(PositiveImbalance::new(value))
		})
	}

	/// Deposit some `value` into the free balance of `who`, possibly creating a new account.
	///
	/// This function is a no-op if:
	/// - the `value` to be deposited is zero; or
	/// - if the `value` to be deposited is less than the ED and the account does not yet exist; or
	/// - `value` is so large it would cause the balance of `who` to overflow.
	fn deposit_creating(
		who: &T::AccountId,
		value: Self::Balance,
	) -> Self::PositiveImbalance {
		if value.is_zero() { return Self::PositiveImbalance::zero() }

		Self::try_mutate_account(who, |account, is_new| -> Result<Self::PositiveImbalance, Self::PositiveImbalance> {
			// bail if not yet created and this operation wouldn't be enough to create it.
			let ed = T::ExistentialDeposit::get();
			ensure!(value >= ed || !is_new, Self::PositiveImbalance::zero());

			// defensive only: overflow should never happen, however in case it does, then this
			// operation is a no-op.
			account.free = account.free.checked_add(&value).ok_or_else(|| Self::PositiveImbalance::zero())?;

			Ok(PositiveImbalance::new(value))
		}).unwrap_or_else(|x| x)
	}

	/// Withdraw some free balance from an account, respecting existence requirements.
	///
	/// Is a no-op if value to be withdrawn is zero.
	fn withdraw(
		who: &T::AccountId,
		value: Self::Balance,
		reasons: WithdrawReasons,
		liveness: ExistenceRequirement,
	) -> result::Result<Self::NegativeImbalance, DispatchError> {
		if value.is_zero() { return Ok(NegativeImbalance::zero()); }

		Self::try_mutate_account(who, |account, _|
			-> Result<Self::NegativeImbalance, DispatchError>
		{
			let new_free_account = account.free.checked_sub(&value)
				.ok_or(Error::<T, I>::InsufficientBalance)?;

			// bail if we need to keep the account alive and this would kill it.
			let ed = T::ExistentialDeposit::get();
			let would_be_dead = new_free_account + account.reserved < ed;
			let would_kill = would_be_dead && account.free + account.reserved >= ed;
			ensure!(liveness == AllowDeath || !would_kill, Error::<T, I>::KeepAlive);

			Self::ensure_can_withdraw(who, value, reasons, new_free_account)?;

			account.free = new_free_account;

			Ok(NegativeImbalance::new(value))
		})
	}

	/// Force the new free balance of a target account `who` to some new value `balance`.
	fn make_free_balance_be(who: &T::AccountId, value: Self::Balance)
		-> SignedImbalance<Self::Balance, Self::PositiveImbalance>
	{
		Self::try_mutate_account(who, |account, is_new|
			-> Result<SignedImbalance<Self::Balance, Self::PositiveImbalance>, ()>
		{
			let ed = T::ExistentialDeposit::get();
			// If we're attempting to set an existing account to less than ED, then
			// bypass the entire operation. It's a no-op if you follow it through, but
			// since this is an instance where we might account for a negative imbalance
			// (in the dust cleaner of set_account) before we account for its actual
			// equal and opposite cause (returned as an Imbalance), then in the
			// instance that there's no other accounts on the system at all, we might
			// underflow the issuance and our arithmetic will be off.
			ensure!(value.saturating_add(account.reserved) >= ed || !is_new, ());

			let imbalance = if account.free <= value {
				SignedImbalance::Positive(PositiveImbalance::new(value - account.free))
			} else {
				SignedImbalance::Negative(NegativeImbalance::new(account.free - value))
			};
			account.free = value;
			Ok(imbalance)
		}).unwrap_or_else(|_| SignedImbalance::Positive(Self::PositiveImbalance::zero()))
	}
}

impl<T: Trait<I>, I: Instance> ReservableCurrency<T::AccountId> for Module<T, I>  where
	T::Balance: MaybeSerializeDeserialize + Debug
{
	/// Check if `who` can reserve `value` from their free balance.
	///
	/// Always `true` if value to be reserved is zero.
	fn can_reserve(who: &T::AccountId, value: Self::Balance) -> bool {
		if value.is_zero() { return true }
		Self::account(who).free
			.checked_sub(&value)
			.map_or(false, |new_balance|
				Self::ensure_can_withdraw(who, value, WithdrawReason::Reserve.into(), new_balance).is_ok()
			)
	}

	fn reserved_balance(who: &T::AccountId) -> Self::Balance {
		Self::account(who).reserved
	}

	/// Move `value` from the free balance from `who` to their reserved balance.
	///
	/// Is a no-op if value to be reserved is zero.
	fn reserve(who: &T::AccountId, value: Self::Balance) -> DispatchResult {
		if value.is_zero() { return Ok(()) }

		Self::try_mutate_account(who, |account, _| -> DispatchResult {
			account.free = account.free.checked_sub(&value).ok_or(Error::<T, I>::InsufficientBalance)?;
			account.reserved = account.reserved.checked_add(&value).ok_or(Error::<T, I>::Overflow)?;
			Self::ensure_can_withdraw(&who, value.clone(), WithdrawReason::Reserve.into(), account.free)
		})?;

		Self::deposit_event(RawEvent::Reserved(who.clone(), value));
		Ok(())
	}

	/// Unreserve some funds, returning any amount that was unable to be unreserved.
	///
	/// Is a no-op if the value to be unreserved is zero.
	fn unreserve(who: &T::AccountId, value: Self::Balance) -> Self::Balance {
		if value.is_zero() { return Zero::zero() }

		let actual = Self::mutate_account(who, |account| {
			let actual = cmp::min(account.reserved, value);
			account.reserved -= actual;
			// defensive only: this can never fail since total issuance which is at least free+reserved
			// fits into the same data type.
			account.free = account.free.saturating_add(actual);
			actual
		});

		Self::deposit_event(RawEvent::Unreserved(who.clone(), actual.clone()));
		value - actual
	}

	/// Slash from reserved balance, returning the negative imbalance created,
	/// and any amount that was unable to be slashed.
	///
	/// Is a no-op if the value to be slashed is zero.
	fn slash_reserved(
		who: &T::AccountId,
		value: Self::Balance
	) -> (Self::NegativeImbalance, Self::Balance) {
		if value.is_zero() { return (NegativeImbalance::zero(), Zero::zero()) }

		Self::mutate_account(who, |account| {
			// underflow should never happen, but it if does, there's nothing to be done here.
			let actual = cmp::min(account.reserved, value);
			account.reserved -= actual;
			(NegativeImbalance::new(actual), value - actual)
		})
	}

	/// Move the reserved balance of one account into the balance of another, according to `status`.
	///
	/// Is a no-op if:
	/// - the value to be moved is zero; or
	/// - the `slashed` id equal to `beneficiary` and the `status` is `Reserved`.
	fn repatriate_reserved(
		slashed: &T::AccountId,
		beneficiary: &T::AccountId,
		value: Self::Balance,
		status: Status,
	) -> Result<Self::Balance, DispatchError> {
		if value.is_zero() { return Ok(Zero::zero()) }

		if slashed == beneficiary {
			return match status {
				Status::Free => Ok(Self::unreserve(slashed, value)),
				Status::Reserved => Ok(value.saturating_sub(Self::reserved_balance(slashed))),
			};
		}

		let actual = Self::try_mutate_account(beneficiary, |to_account, is_new|-> Result<Self::Balance, DispatchError> {
			ensure!(!is_new, Error::<T, I>::DeadAccount);
			Self::try_mutate_account(slashed, |from_account, _| -> Result<Self::Balance, DispatchError> {
				let actual = cmp::min(from_account.reserved, value);
				match status {
					Status::Free => to_account.free = to_account.free.checked_add(&actual).ok_or(Error::<T, I>::Overflow)?,
					Status::Reserved => to_account.reserved = to_account.reserved.checked_add(&actual).ok_or(Error::<T, I>::Overflow)?,
				}
				from_account.reserved -= actual;
				Ok(actual)
			})
		})?;

		Self::deposit_event(RawEvent::ReserveRepatriated(slashed.clone(), beneficiary.clone(), actual, status));
		Ok(value - actual)
	}
}

/// Implement `OnKilledAccount` to remove the local account, if using local account storage.
///
/// NOTE: You probably won't need to use this! This only needs to be "wired in" to System module
/// if you're using the local balance storage. **If you're using the composite system account
/// storage (which is the default in most examples and tests) then there's no need.**
impl<T: Trait<I>, I: Instance> OnKilledAccount<T::AccountId> for Module<T, I> {
	fn on_killed_account(who: &T::AccountId) {
		Account::<T, I>::mutate_exists(who, |account| {
			let total = account.as_ref().map(|acc| acc.total()).unwrap_or_default();
			if !total.is_zero() {
				T::DustRemoval::on_unbalanced(NegativeImbalance::new(total));
				Self::deposit_event(RawEvent::DustLost(who.clone(), total));
			}
			*account = None;
		});
	}
}

impl<T: Trait<I>, I: Instance> LockableCurrency<T::AccountId> for Module<T, I>
where
	T::Balance: MaybeSerializeDeserialize + Debug
{
	type Moment = T::BlockNumber;

	type MaxLocks = T::MaxLocks;

	// Set a lock on the balance of `who`.
	// Is a no-op if lock amount is zero or `reasons` `is_none()`.
	fn set_lock(
		id: LockIdentifier,
		who: &T::AccountId,
		amount: T::Balance,
		reasons: WithdrawReasons,
	) {
		if amount.is_zero() || reasons.is_none() { return }
		let mut new_lock = Some(BalanceLock { id, amount, reasons: reasons.into() });
		let mut locks = Self::locks(who).into_iter()
			.filter_map(|l| if l.id == id { new_lock.take() } else { Some(l) })
			.collect::<Vec<_>>();
		if let Some(lock) = new_lock {
			locks.push(lock)
		}
		Self::update_locks(who, &locks[..]);
	}

	// Extend a lock on the balance of `who`.
	// Is a no-op if lock amount is zero or `reasons` `is_none()`.
	fn extend_lock(
		id: LockIdentifier,
		who: &T::AccountId,
		amount: T::Balance,
		reasons: WithdrawReasons,
	) {
		if amount.is_zero() || reasons.is_none() { return }
		let mut new_lock = Some(BalanceLock { id, amount, reasons: reasons.into() });
		let mut locks = Self::locks(who).into_iter().filter_map(|l|
			if l.id == id {
				new_lock.take().map(|nl| {
					BalanceLock {
						id: l.id,
						amount: l.amount.max(nl.amount),
						reasons: l.reasons | nl.reasons,
					}
				})
			} else {
				Some(l)
			}).collect::<Vec<_>>();
		if let Some(lock) = new_lock {
			locks.push(lock)
		}
		Self::update_locks(who, &locks[..]);
	}

	fn remove_lock(
		id: LockIdentifier,
		who: &T::AccountId,
	) {
		let mut locks = Self::locks(who);
		locks.retain(|l| l.id != id);
		Self::update_locks(who, &locks[..]);
	}
}

impl<T: Trait<I>, I: Instance> IsDeadAccount<T::AccountId> for Module<T, I> where
	T::Balance: MaybeSerializeDeserialize + Debug
{
	fn is_dead_account(who: &T::AccountId) -> bool {
		// this should always be exactly equivalent to `Self::account(who).total().is_zero()` if ExistentialDeposit > 0
		!T::AccountStore::is_explicit(who)
	}
}