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@ -8,7 +8,7 @@ implemented and reviewed via the normal GitHub pull request workflow.
Some changes though are "substantial", and we ask that these be put Some changes though are "substantial", and we ask that these be put
through a bit of a design process and produce a consensus among the Rust through a bit of a design process and produce a consensus among the Rust
community and the [core team]. community and the [sub-team]s.
The "RFC" (request for comments) process is intended to provide a The "RFC" (request for comments) process is intended to provide a
consistent and controlled path for new features to enter the language consistent and controlled path for new features to enter the language
@ -80,19 +80,21 @@ the direction the language is evolving in.
* [RFC Postponement] * [RFC Postponement]
* [Help this is all too informal!] * [Help this is all too informal!]
## When you need to follow this process ## When you need to follow this process
[When you need to follow this process]: #when-you-need-to-follow-this-process [When you need to follow this process]: #when-you-need-to-follow-this-process
You need to follow this process if you intend to make "substantial" You need to follow this process if you intend to make "substantial" changes to
changes to Rust, Cargo, Crates.io, or the RFC process itself. What constitutes Rust, Cargo, Crates.io, or the RFC process itself. What constitutes a
a "substantial" change is evolving based on community norms, but may include "substantial" change is evolving based on community norms and varies depending
the following. on what part of the ecosystem you are proposing to change, but may include the
following.
- Any semantic or syntactic change to the language that is not a bugfix. - Any semantic or syntactic change to the language that is not a bugfix.
- Removing language features, including those that are feature-gated. - Removing language features, including those that are feature-gated.
- Changes to the interface between the compiler and libraries, - Changes to the interface between the compiler and libraries, including lang
including lang items and intrinsics. items and intrinsics.
- Additions to `std` - Additions to `std`.
Some changes do not require an RFC: Some changes do not require an RFC:
@ -108,6 +110,15 @@ If you submit a pull request to implement a new feature without going
through the RFC process, it may be closed with a polite request to through the RFC process, it may be closed with a polite request to
submit an RFC first. submit an RFC first.
For more details on when an RFC is required, please see the following specific
guidelines, these correspond with some of the Rust community's
[sub-teams](http://www.rust-lang.org/team.html):
* [language changes](lang_changes.md),
* [library changes](libs_changes.md),
* [compiler changes](compiler_changes.md).
## Before creating an RFC ## Before creating an RFC
[Before creating an RFC]: #before-creating-an-rfc [Before creating an RFC]: #before-creating-an-rfc
@ -130,12 +141,12 @@ on the [RFC issue tracker][issues], and occasionally posting
review. review.
As a rule of thumb, receiving encouraging feedback from long-standing As a rule of thumb, receiving encouraging feedback from long-standing
project developers, and particularly members of the [core team][core] project developers, and particularly members of the relevant [sub-team]
is a good indication that the RFC is worth pursuing. is a good indication that the RFC is worth pursuing.
[issues]: https://github.com/rust-lang/rfcs/issues [issues]: https://github.com/rust-lang/rfcs/issues
[discuss]: http://discuss.rust-lang.org/ [discuss]: http://discuss.rust-lang.org/
[core]: https://github.com/rust-lang/rust/wiki/Note-core-team
## What the process is ## What the process is
[What the process is]: #what-the-process-is [What the process is]: #what-the-process-is
@ -146,49 +157,57 @@ is 'active' and may be implemented with the goal of eventual inclusion
into Rust. into Rust.
* Fork the RFC repo http://github.com/rust-lang/rfcs * Fork the RFC repo http://github.com/rust-lang/rfcs
* Copy `0000-template.md` to `text/0000-my-feature.md` (where * Copy `0000-template.md` to `text/0000-my-feature.md` (where 'my-feature' is
'my-feature' is descriptive. don't assign an RFC number yet). descriptive. don't assign an RFC number yet).
* Fill in the RFC. Put care into the details: RFCs that do not * Fill in the RFC. Put care into the details: RFCs that do not present
present convincing motivation, demonstrate understanding of the convincing motivation, demonstrate understanding of the impact of the design, or
impact of the design, or are disingenuous about the drawbacks or are disingenuous about the drawbacks or alternatives tend to be poorly-received.
alternatives tend to be poorly-received. * Submit a pull request. As a pull request the RFC will receive design feedback
* Submit a pull request. As a pull request the RFC will receive design from the larger community, and the author should be prepared to revise it in
feedback from the larger community, and the author should be prepared response.
to revise it in response. * Each pull request will be labeled with the most relevant [sub-team].
* During Rust triage, the pull request will either be closed (for RFCs * Each sub-team triages its RFC PRs. The sub-team will will either close the PR
that clearly will not be accepted) or assigned a *shepherd*. The (for RFCs that clearly will not be accepted) or assign it a *shepherd*. The
shepherd is a trusted developer who is familiar with the process, who shepherd is a trusted developer who is familiar with the RFC process, who will
will help to move the RFC forward, and ensure that the right people help to move the RFC forward, and ensure that the right people see and review
see and review it. it.
* Build consensus and integrate feedback. RFCs that have broad support * Build consensus and integrate feedback. RFCs that have broad support are much
are much more likely to make progress than those that don't receive more likely to make progress than those that don't receive any comments. The
any comments. The shepherd assigned to your RFC should help you get shepherd assigned to your RFC should help you get feedback from Rust developers
feedback from Rust developers as well. as well.
* The shepherd may schedule meetings with the author and/or relevant * The shepherd may schedule meetings with the author and/or relevant
stakeholders to discuss the issues in greater detail, and in some stakeholders to discuss the issues in greater detail.
cases the topic may be discussed at the larger [weekly meeting]. In * The sub-team will discuss the RFC PR, as much as possible in the comment
either case a summary from the meeting will be posted back to the RFC thread of the PR itself. Offline discussion will be summarized on the PR comment
pull request. thread.
* Once both proponents and opponents have clarified and defended * Once both proponents and opponents have clarified and defended positions and
positions and the conversation has settled, the shepherd will take it the conversation has settled, the RFC will enter its *final comment period*
to the [core team] for a final decision. (FCP). This is a final opportunity for the community to comment on the PR and is
* Eventually, someone from the [core team] will either accept the RFC a reminder for all members of the sub-team to be aware of the RFC.
by merging the pull request, assigning the RFC a number (corresponding * The FCP lasts one week. It may be extended if consensus between sub-team
to the pull request number), at which point the RFC is 'active', or members cannot be reached. At the end of the FCP, the [sub-team] will either
reject it by closing the pull request. accept the RFC by merging the pull request, assigning the RFC a number
(corresponding to the pull request number), at which point the RFC is 'active',
or reject it by closing the pull request. How exactly the sub-team decide on an
RFC is up to the sub-team.
## The role of the shepherd ## The role of the shepherd
[The role of the shepherd]: #the-role-of-the-shepherd [The role of the shepherd]: #the-role-of-the-shepherd
During triage, every RFC will either be closed or assigned a shepherd. During triage, every RFC will either be closed or assigned a shepherd from the
The role of the shepherd is to move the RFC through the process. This relevant sub-team. The role of the shepherd is to move the RFC through the
starts with simply reading the RFC in detail and providing initial process. This starts with simply reading the RFC in detail and providing initial
feedback. The shepherd should also solicit feedback from people who feedback. The shepherd should also solicit feedback from people who are likely
are likely to have strong opinions about the RFC. Finally, when this to have strong opinions about the RFC. When this feedback has been incorporated
feedback has been incorporated and the RFC seems to be in a steady and the RFC seems to be in a steady state, the shepherd and/or sub-team leader
state, the shepherd will bring it to the meeting. In general, the idea will announce an FCP. In general, the idea here is to "front-load" as much of
here is to "front-load" as much of the feedback as possible before the the feedback as possible before the point where we actually reach a decision -
point where we actually reach a decision. by the end of the FCP, the decision on whether or not to accept the RFC should
usually be obvious from the RFC discussion thread. On occasion, there may not be
consensus but discussion has stalled. In this case, the relevant team will make
a decision.
## The RFC life-cycle ## The RFC life-cycle
[The RFC life-cycle]: #the-rfc-life-cycle [The RFC life-cycle]: #the-rfc-life-cycle
@ -210,35 +229,36 @@ through to completion: authors should not expect that other project
developers will take on responsibility for implementing their accepted developers will take on responsibility for implementing their accepted
feature. feature.
Modifications to active RFC's can be done in followup PR's. We strive Modifications to active RFC's can be done in follow-up PR's. We strive
to write each RFC in a manner that it will reflect the final design of to write each RFC in a manner that it will reflect the final design of
the feature; but the nature of the process means that we cannot expect the feature; but the nature of the process means that we cannot expect
every merged RFC to actually reflect what the end result will be at every merged RFC to actually reflect what the end result will be at
the time of the next major release; therefore we try to keep each RFC the time of the next major release.
document somewhat in sync with the language feature as planned,
tracking such changes via followup pull requests to the document. In general, once accepted, RFCs should not be substantially changed. Only very
minor changes should be submitted as amendments. More substantial changes should
be new RFCs, with a note added to the original RFC. Exactly what counts as a
"very minor change" is up to the sub-team to decide. There are some more
specific guidelines in the sub-team RFC guidelines for the [language](lang_changes.md),
[libraries](libs_changes.md), and [compiler](compiler_changes.md).
An RFC that makes it through the entire process to implementation is
considered 'complete' and is moved to the 'complete' folder; an RFC
that fails after becoming active is 'inactive' and moves to the
'inactive' folder.
## Reviewing RFC's ## Reviewing RFC's
[Reviewing RFC's]: #reviewing-rfcs [Reviewing RFC's]: #reviewing-rfcs
While the RFC PR is up, the shepherd may schedule meetings with the While the RFC PR is up, the shepherd may schedule meetings with the
author and/or relevant stakeholders to discuss the issues in greater author and/or relevant stakeholders to discuss the issues in greater
detail, and in some cases the topic may be discussed at the larger detail, and in some cases the topic may be discussed at a sub-team
[weekly meeting]. In either case a summary from the meeting will be meeting. In either case a summary from the meeting will be
posted back to the RFC pull request. posted back to the RFC pull request.
The core team makes final decisions about RFCs after the benefits and A sub-team makes final decisions about RFCs after the benefits and drawbacks are
drawbacks are well understood. These decisions can be made at any well understood. These decisions can be made at any time, but the sub-team will
time, but the core team will regularly issue decisions on at least a regularly issue decisions. When a decision is made, the RFC PR will either be
weekly basis. When a decision is made, the RFC PR will either be merged or closed. In either case, if the reasoning is not clear from the
merged or closed, in either case with a comment describing the discussion in thread, the sub-team will add a comment describing the rationale
rationale for the decision. The comment should largely be a summary of for the decision.
discussion already on the comment thread.
## Implementing an RFC ## Implementing an RFC
[Implementing an RFC]: #implementing-an-rfc [Implementing an RFC]: #implementing-an-rfc
@ -248,7 +268,7 @@ implemented right away. Other accepted RFC's can represent features
that can wait until some arbitrary developer feels like doing the that can wait until some arbitrary developer feels like doing the
work. Every accepted RFC has an associated issue tracking its work. Every accepted RFC has an associated issue tracking its
implementation in the Rust repository; thus that associated issue can implementation in the Rust repository; thus that associated issue can
be assigned a priority via the [triage process] that the team uses for be assigned a priority via the triage process that the team uses for
all issues in the Rust repository. all issues in the Rust repository.
The author of an RFC is not obligated to implement it. Of course, the The author of an RFC is not obligated to implement it. Of course, the
@ -259,15 +279,18 @@ If you are interested in working on the implementation for an 'active'
RFC, but cannot determine if someone else is already working on it, RFC, but cannot determine if someone else is already working on it,
feel free to ask (e.g. by leaving a comment on the associated issue). feel free to ask (e.g. by leaving a comment on the associated issue).
## RFC Postponement ## RFC Postponement
[RFC Postponement]: #rfc-postponement [RFC Postponement]: #rfc-postponement
Some RFC pull requests are tagged with the 'postponed' label when they Some RFC pull requests are tagged with the 'postponed' label when they are
are closed (as part of the rejection process). An RFC closed with closed (as part of the rejection process). An RFC closed with “postponed” is
“postponed” is marked as such because we want neither to think about marked as such because we want neither to think about evaluating the proposal
evaluating the proposal nor about implementing the described feature nor about implementing the described feature until some time in the future, and
until after the next major release, and we believe that we can afford we believe that we can afford to wait until then to do so. Historically,
to wait until then to do so. "postponed" was used to postpone features until after 1.0. Postponed PRs may be
re-opened when the time is right. We don't have any formal process for that, you
should ask members of the relevant sub-team.
Usually an RFC pull request marked as “postponed” has already passed Usually an RFC pull request marked as “postponed” has already passed
an informal first round of evaluation, namely the round of “do we an informal first round of evaluation, namely the round of “do we
@ -285,6 +308,4 @@ present circumstances. As usual, we are trying to let the process be
driven by consensus and community norms, not impose more structure than driven by consensus and community norms, not impose more structure than
necessary. necessary.
[core team]: https://github.com/mozilla/rust/wiki/Note-core-team [sub-team]: http://www.rust-lang.org/team.html
[triage process]: https://github.com/rust-lang/rust/wiki/Note-development-policy#milestone-and-priority-nomination-and-triage
[weekly meeting]: https://github.com/rust-lang/meeting-minutes

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@ -0,0 +1,52 @@
# RFC policy - the compiler
We have not previously had an RFC system for compiler changes, so policy here is
likely to change as we get the hang of things. We don't want to slow down most
compiler development, but on the other hand we do want to do more design work
ahead of time on large additions and refactorings.
Compiler RFCs will be managed by the compiler sub-team, and tagged `T-compiler`.
The compiler sub-team will do an initial triage of new PRs within a week of
submission. The result of triage will either be that the PR is assigned to a
member of the sub-team for shepherding, the PR is closed because the sub-team
believe it should be done without an RFC, or closed because the sub-team feel it
should clearly not be done and further discussion is not necessary. We'll follow
the standard procedure for shepherding, final comment period, etc.
Where there is significant design work for the implementation of a language
feature, the preferred workflow is to submit two RFCs - one for the language
design and one for the implementation design. The implementation RFC may be
submitted later if there is scope for large changes to the language RFC.
## Changes which need an RFC
* Large refactorings or redesigns of the compiler
* Changing the API presented to syntax extensions or other compiler plugins in
non-trivial ways
* Adding, removing, or changing a stable compiler flag
* The implementation of new language features where there is significant change
or addition to the compiler. There is obviously some room for interpretation
about what consitutes a "significant" change and how much detail the
implementation RFC needs. For guidance, [associated items](text/0195-associated-items.md)
and [UFCS](text/0132-ufcs.md) would clearly need an implementation RFC,
[type ascription](text/0803-type-ascription.md) and
[lifetime elision](text/0141-lifetime-elision.md) would not.
* Any other change which causes backwards incompatible changes to stable
behaviour of the compiler, language, or libraries
## Changes which don't need an RFC
* Bug fixes, improved error messages, etc.
* Minor refactoring/tidying up
* Implmenting language features which have an accepted RFC, where the
implementation does not significantly change the compiler or require
significant new design work
* Adding unstable API for tools (note that all compiler API is currently unstable)
* Adding, removing, or changing an unstable compiler flag (if the compiler flag
is widely used there should be at least some discussion on discuss, or an RFC
in some cases)
If in doubt it is probably best to just announce the change you want to make to
the compiler subteam on discuss or IRC, and see if anyone feels it needs an RFC.

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@ -0,0 +1,36 @@
# RFC policy - language design
Pretty much every change to the language needs an RFC.
Language RFCs are managed by the language sub-team, and tagged `T-lang`. The
language sub-team will do an initial triage of new PRs within a week of
submission. The result of triage will either be that the PR is assigned to a
member of the sub-team for shepherding, the PR is closed as postponed because
the subteam believe it might be a good idea, but is not currently aligned with
Rust's priorities, or the PR is closed because the sub-team feel it should
clearly not be done and further discussion is not necessary. In the latter two
cases, the sub-team will give a detailed explanation. We'll follow the standard
procedure for shepherding, final comment period, etc.
## Amendments
Sometimes in the implementation of an RFC, changes are required. In general
these don't require an RFC as long as they are very minor and in the spirit of
the accepted RFC (essentially bug fixes). In this case implementers should
submit an RFC PR which amends the accepted RFC with the new details. Although
the RFC repository is not intended as a reference manual, it is preferred that
RFCs do reflect what was actually implemented. Amendment RFCs will go through
the same process as regular RFCs, but should be less controversial and thus
should move more quickly.
When a change is more dramatic, it is better to create a new RFC. The RFC should
be standalone and reference the original, rather than modifying the existing
RFC. You should add a comment to the original RFC with referencing the new RFC
as part of the PR.
Obviously there is some scope for judgment here. As a guideline, if a change
affects more than one part of the RFC (i.e., is a non-local change), affects the
applicability of the RFC to its motivating use cases, or there are multiple
possible new solutions, then the feature is probably not 'minor' and should get
a new RFC.

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@ -0,0 +1,114 @@
# RFC guidelines - libraries sub-team
# Motivation
* RFCs are heavyweight:
* RFCs generally take at minimum 2 weeks from posting to land. In
practice it can be more on the order of months for particularly
controversial changes.
* RFCs are a lot of effort to write; especially for non-native speakers or
for members of the community whose strengths are more technical than literary.
* RFCs may involve pre-RFCs and several rewrites to accommodate feedback.
* RFCs require a dedicated shepherd to herd the community and author towards
consensus.
* RFCs require review from a majority of the subteam, as well as an official
vote.
* RFCs can't be downgraded based on their complexity. Full process always applies.
Easy RFCs may certainly land faster, though.
* RFCs can be very abstract and hard to grok the consequences of (no implementation).
* PRs are low *overhead* but potentially expensive nonetheless:
* Easy PRs can get insta-merged by any rust-lang contributor.
* Harder PRs can be easily escalated. You can ping subject-matter experts for second
opinions. Ping the whole team!
* Easier to grok the full consequences. Lots of tests and Crater to save the day.
* PRs can be accepted optimistically with bors, buildbot, and the trains to guard
us from major mistakes making it into stable. The size of the nightly community
at this point in time can still mean major community breakage regardless of trains,
however.
* HOWEVER: Big PRs can be a lot of work to make only to have that work rejected for
details that could have been hashed out first.
* RFCs are *only* meaningful if a significant and diverse portion of the
community actively participates in them. The official teams are not
sufficiently diverse to establish meaningful community consensus by agreeing
amongst themselves.
* If there are *tons* of RFCs -- especially trivial ones -- people are less
likely to engage with them. Official team members are super busy. Domain experts
and industry professionals are super busy *and* have no responsibility to engage
in RFCs. Since these are *exactly* the most important people to get involved in
the RFC process, it is important that we be maximally friendly towards their
needs.
# Is an RFC required?
The overarching philosophy is: *do whatever is easiest*. If an RFC
would be less work than an implementation, that's a good sign that an RFC is
necessary. That said, if you anticipate controversy, you might want to short-circuit
straight to an RFC. For instance new APIs almost certainly merit an RFC. Especially
as `std` has become more conservative in favour of the much more agile cargoverse.
* **Submit a PR** if the change is a:
* Bugfix
* Docfix
* Obvious API hole patch, such as adding an API from one type to a symmetric type.
e.g. `Vec<T> -> Box<[T]>` clearly motivates adding `String -> Box<str>`
* Minor tweak to an unstable API (renaming, generalizing)
* Implementing an "obvious" trait like Clone/Debug/etc
* **Submit an RFC** if the change is a:
* New API
* Semantic Change to a stable API
* Generalization of a stable API (e.g. how we added Pattern or Borrow)
* Deprecation of a stable API
* Nontrivial trait impl (because all trait impls are insta-stable)
* **Do the easier thing** if uncertain. (choosing a path is not final)
# Non-RFC process
* A (non-RFC) PR is likely to be **closed** if clearly not acceptable:
* Disproportionate breaking change (small inference breakage may be acceptable)
* Unsound
* Doesn't fit our general design philosophy around the problem
* Better as a crate
* Too marginal for std
* Significant implementation problems
* A PR may also be closed because an RFC is approriate.
* A (non-RFC) PR may be **merged as unstable**. In this case, the feature
should have a fresh feature gate and an associated tracking issue for
stabilisation. Note that trait impls and docs are insta-stable and thus have no
tracking issue. This may imply requiring a higher level of scrutiny for such
changes.
However, an accepted RFC is not a rubber-stamp for merging an implementation PR.
Nor must an implementation PR perfectly match the RFC text. Implementation details
may merit deviations, though obviously they should be justified. The RFC may be
amended if deviations are substantial, but are not generally necessary. RFCs should
favour immutability. The RFC + Issue + PR should form a total explanation of the
current implementation.
* Once something has been merged as unstable, a shepherd should be assigned
to promote and obtain feedback on the design.
* Every time a release cycle ends, the libs teams assesses the current unstable
APIs and selects some number of them for potential stabilization during the
next cycle. These are announced for FCP at the beginning of the cycle, and
(possibly) stabilized just before the beta is cut.
* After the final comment period, an API should ideally take one of two paths:
* **Stabilize** if the change is desired, and consensus is reached
* **Deprecate** is the change is undesired, and consensus is reached
* **Extend the FCP** is the change cannot meet consensus
* If consensus *still* can't be reached, consider requiring a new RFC or
just deprecating as "too controversial for std".
* If any problems are found with a newly stabilized API during its beta period,
*strongly* favour reverting stability in order to prevent stabilizing a bad
API. Due to the speed of the trains, this is not a serious delay (~2-3 months
if it's not a major problem).

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@ -125,10 +125,15 @@ The error conditions that can arise, and their defined results, are as
follows. The intention is that the defined results are the same as the follows. The intention is that the defined results are the same as the
defined results today. The only change is that now a panic may result. defined results today. The only change is that now a panic may result.
- The operations `+`, `-`, `*`, `/`, `%` can underflow and - The operations `+`, `-`, `*`, can underflow and overflow. When checking is
overflow. enabled this will panic. When checking is disabled this will two's complement
- Shift operations (`<<`, `>>`) can shift a value of width `N` by more wrap.
than `N` bits. - The operations `/`, `%` for the arguments `INT_MIN` and `-1`
will unconditionally panic. This is unconditional for legacy reasons.
- Shift operations (`<<`, `>>`) on a value of with `N` can be passed a shift value
>= `N`. It is unclear what behaviour should result from this, so the shift value
is unconditionally masked to be modulo `N` to ensure that the argument is always
in range.
## Enabling overflow checking ## Enabling overflow checking
@ -145,7 +150,7 @@ potential overflow (and, in particular, for code where overflow is
expected and normal, they will be immediately guided to use the expected and normal, they will be immediately guided to use the
wrapping methods introduced below). However, because these checks will wrapping methods introduced below). However, because these checks will
be compiled out whenever an optimized build is produced, final code be compiled out whenever an optimized build is produced, final code
wilil not pay a performance penalty. will not pay a performance penalty.
In the future, we may add additional means to control when overflow is In the future, we may add additional means to control when overflow is
checked, such as scoped attributes or a global, independent checked, such as scoped attributes or a global, independent
@ -451,17 +456,7 @@ were:
# Unresolved questions # Unresolved questions
The C semantics of wrapping operations in some cases are undefined: None today (see Updates section below).
- `INT_MIN / -1`, `INT_MIN % -1`
- Shifts by an excessive number of bits
This RFC takes no position on the correct semantics of these
operations, simply preserving the existing semantics. However, it may
be worth trying to define the wrapping semantics of these operations
in a portable way, even if that implies some runtime cost. Since these
are all error conditions, this is an orthogonal topic to the matter of
overflow.
# Future work # Future work
@ -491,6 +486,10 @@ Since it was accepted, the RFC has been updated as follows:
2. `as` was changed to restore the behavior before the RFC (that is, 2. `as` was changed to restore the behavior before the RFC (that is,
it truncates to the target bitwidth and reinterprets the highest it truncates to the target bitwidth and reinterprets the highest
order bit, a.k.a. sign-bit, as necessary, as a C cast would). order bit, a.k.a. sign-bit, as necessary, as a C cast would).
3. Shifts were specified to mask off the bits of over-long shifts.
4. Overflow was specified to be two's complement wrapping (this was mostly
a clarification).
5. `INT_MIN / -1` and `INT_MIN % -1` panics.
# Acknowledgements and further reading # Acknowledgements and further reading