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33 files changed, 3021 insertions, 173 deletions
diff --git a/Documentation/Makefile b/Documentation/Makefile index cd5b4396db..3f599524ea 100644 --- a/Documentation/Makefile +++ b/Documentation/Makefile @@ -17,6 +17,7 @@ DOC_HTML=$(MAN_HTML) ARTICLES = howto-index ARTICLES += everyday ARTICLES += git-tools +ARTICLES += git-bisect-lk2009 # with their own formatting rules. SP_ARTICLES = howto/revert-branch-rebase howto/using-merge-subtree user-manual API_DOCS = $(patsubst %.txt,%,$(filter-out technical/api-index-skel.txt technical/api-index.txt, $(wildcard technical/api-*.txt))) diff --git a/Documentation/config.txt b/Documentation/config.txt index a8e0876a2a..a1e36d7e42 100644 --- a/Documentation/config.txt +++ b/Documentation/config.txt @@ -635,10 +635,10 @@ color.diff.<slot>:: Use customized color for diff colorization. `<slot>` specifies which part of the patch to use the specified color, and is one of `plain` (context text), `meta` (metainformation), `frag` - (hunk header), `old` (removed lines), `new` (added lines), - `commit` (commit headers), or `whitespace` (highlighting - whitespace errors). The values of these variables may be specified as - in color.branch.<slot>. + (hunk header), 'func' (function in hunk header), `old` (removed lines), + `new` (added lines), `commit` (commit headers), or `whitespace` + (highlighting whitespace errors). The values of these variables may be + specified as in color.branch.<slot>. color.grep:: When set to `always`, always highlight matches. When `false` (or diff --git a/Documentation/git-bisect-lk2009.txt b/Documentation/git-bisect-lk2009.txt new file mode 100644 index 0000000000..6b7b2e5497 --- /dev/null +++ b/Documentation/git-bisect-lk2009.txt @@ -0,0 +1,1358 @@ +Fighting regressions with git bisect +==================================== +:Author: Christian Couder +:Email: chriscool@tuxfamily.org +:Date: 2009/11/08 + +Abstract +-------- + +"git bisect" enables software users and developers to easily find the +commit that introduced a regression. We show why it is important to +have good tools to fight regressions. We describe how "git bisect" +works from the outside and the algorithms it uses inside. Then we +explain how to take advantage of "git bisect" to improve current +practices. And we discuss how "git bisect" could improve in the +future. + + +Introduction to "git bisect" +---------------------------- + +Git is a Distributed Version Control system (DVCS) created by Linus +Torvalds and maintained by Junio Hamano. + +In Git like in many other Version Control Systems (VCS), the different +states of the data that is managed by the system are called +commits. And, as VCS are mostly used to manage software source code, +sometimes "interesting" changes of behavior in the software are +introduced in some commits. + +In fact people are specially interested in commits that introduce a +"bad" behavior, called a bug or a regression. They are interested in +these commits because a commit (hopefully) contains a very small set +of source code changes. And it's much easier to understand and +properly fix a problem when you only need to check a very small set of +changes, than when you don't know where look in the first place. + +So to help people find commits that introduce a "bad" behavior, the +"git bisect" set of commands was invented. And it follows of course +that in "git bisect" parlance, commits where the "interesting +behavior" is present are called "bad" commits, while other commits are +called "good" commits. And a commit that introduce the behavior we are +interested in is called a "first bad commit". Note that there could be +more than one "first bad commit" in the commit space we are searching. + +So "git bisect" is designed to help find a "first bad commit". And to +be as efficient as possible, it tries to perform a binary search. + + +Fighting regressions overview +----------------------------- + +Regressions: a big problem +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Regressions are a big problem in the software industry. But it's +difficult to put some real numbers behind that claim. + +There are some numbers about bugs in general, like a NIST study in +2002 <<1>> that said: + +_____________ +Software bugs, or errors, are so prevalent and so detrimental that +they cost the U.S. economy an estimated $59.5 billion annually, or +about 0.6 percent of the gross domestic product, according to a newly +released study commissioned by the Department of Commerce's National +Institute of Standards and Technology (NIST). At the national level, +over half of the costs are borne by software users and the remainder +by software developers/vendors. The study also found that, although +all errors cannot be removed, more than a third of these costs, or an +estimated $22.2 billion, could be eliminated by an improved testing +infrastructure that enables earlier and more effective identification +and removal of software defects. These are the savings associated with +finding an increased percentage (but not 100 percent) of errors closer +to the development stages in which they are introduced. Currently, +over half of all errors are not found until "downstream" in the +development process or during post-sale software use. +_____________ + +And then: + +_____________ +Software developers already spend approximately 80 percent of +development costs on identifying and correcting defects, and yet few +products of any type other than software are shipped with such high +levels of errors. +_____________ + +Eventually the conclusion started with: + +_____________ +The path to higher software quality is significantly improved software +testing. +_____________ + +There are other estimates saying that 80% of the cost related to +software is about maintenance <<2>>. + +Though, according to Wikipedia <<3>>: + +_____________ +A common perception of maintenance is that it is merely fixing +bugs. However, studies and surveys over the years have indicated that +the majority, over 80%, of the maintenance effort is used for +non-corrective actions (Pigosky 1997). This perception is perpetuated +by users submitting problem reports that in reality are functionality +enhancements to the system. +_____________ + +But we can guess that improving on existing software is very costly +because you have to watch out for regressions. At least this would +make the above studies consistent among themselves. + +Of course some kind of software is developed, then used during some +time without being improved on much, and then finally thrown away. In +this case, of course, regressions may not be a big problem. But on the +other hand, there is a lot of big software that is continually +developed and maintained during years or even tens of years by a lot +of people. And as there are often many people who depend (sometimes +critically) on such software, regressions are a really big problem. + +One such software is the linux kernel. And if we look at the linux +kernel, we can see that a lot of time and effort is spent to fight +regressions. The release cycle start with a 2 weeks long merge +window. Then the first release candidate (rc) version is tagged. And +after that about 7 or 8 more rc versions will appear with around one +week between each of them, before the final release. + +The time between the first rc release and the final release is +supposed to be used to test rc versions and fight bugs and especially +regressions. And this time is more than 80% of the release cycle +time. But this is not the end of the fight yet, as of course it +continues after the release. + +And then this is what Ingo Molnar (a well known linux kernel +developer) says about his use of git bisect: + +_____________ +I most actively use it during the merge window (when a lot of trees +get merged upstream and when the influx of bugs is the highest) - and +yes, there have been cases that i used it multiple times a day. My +average is roughly once a day. +_____________ + +So regressions are fought all the time by developers, and indeed it is +well known that bugs should be fixed as soon as possible, so as soon +as they are found. That's why it is interesting to have good tools for +this purpose. + +Other tools to fight regressions +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +So what are the tools used to fight regressions? They are nearly the +same as those used to fight regular bugs. The only specific tools are +test suites and tools similar as "git bisect". + +Test suites are very nice. But when they are used alone, they are +supposed to be used so that all the tests are checked after each +commit. This means that they are not very efficient, because many +tests are run for no interesting result, and they suffer from +combinational explosion. + +In fact the problem is that big software often has many different +configuration options and that each test case should pass for each +configuration after each commit. So if you have for each release: N +configurations, M commits and T test cases, you should perform: + +------------- +N * M * T tests +------------- + +where N, M and T are all growing with the size your software. + +So very soon it will not be possible to completely test everything. + +And if some bugs slip through your test suite, then you can add a test +to your test suite. But if you want to use your new improved test +suite to find where the bug slipped in, then you will either have to +emulate a bisection process or you will perhaps bluntly test each +commit backward starting from the "bad" commit you have which may be +very wasteful. + +"git bisect" overview +--------------------- + +Starting a bisection +~~~~~~~~~~~~~~~~~~~~ + +The first "git bisect" subcommand to use is "git bisect start" to +start the search. Then bounds must be set to limit the commit +space. This is done usually by giving one "bad" and at least one +"good" commit. They can be passed in the initial call to "git bisect +start" like this: + +------------- +$ git bisect start [BAD [GOOD...]] +------------- + +or they can be set using: + +------------- +$ git bisect bad [COMMIT] +------------- + +and: + +------------- +$ git bisect good [COMMIT...] +------------- + +where BAD, GOOD and COMMIT are all names that can be resolved to a +commit. + +Then "git bisect" will checkout a commit of its choosing and ask the +user to test it, like this: + +------------- +$ git bisect start v2.6.27 v2.6.25 +Bisecting: 10928 revisions left to test after this (roughly 14 steps) +[2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit +------------- + +Note that the example that we will use is really a toy example, we +will be looking for the first commit that has a version like +"2.6.26-something", that is the commit that has a "SUBLEVEL = 26" line +in the top level Makefile. This is a toy example because there are +better ways to find this commit with git than using "git bisect" (for +example "git blame" or "git log -S<string>"). + +Driving a bisection manually +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +At this point there are basically 2 ways to drive the search. It can +be driven manually by the user or it can be driven automatically by a +script or a command. + +If the user is driving it, then at each step of the search, the user +will have to test the current commit and say if it is "good" or "bad" +using the "git bisect good" or "git bisect bad" commands respectively +that have been described above. For example: + +------------- +$ git bisect bad +Bisecting: 5480 revisions left to test after this (roughly 13 steps) +[66c0b394f08fd89236515c1c84485ea712a157be] KVM: kill file->f_count abuse in kvm +------------- + +And after a few more steps like that, "git bisect" will eventually +find a first bad commit: + +------------- +$ git bisect bad +2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit +commit 2ddcca36c8bcfa251724fe342c8327451988be0d +Author: Linus Torvalds <torvalds@linux-foundation.org> +Date: Sat May 3 11:59:44 2008 -0700 + + Linux 2.6.26-rc1 + +:100644 100644 5cf8258195331a4dbdddff08b8d68642638eea57 4492984efc09ab72ff6219a7bc21fb6a957c4cd5 M Makefile +------------- + +At this point we can see what the commit does, check it out (if it's +not already checked out) or tinker with it, for example: + +------------- +$ git show HEAD +commit 2ddcca36c8bcfa251724fe342c8327451988be0d +Author: Linus Torvalds <torvalds@linux-foundation.org> +Date: Sat May 3 11:59:44 2008 -0700 + + Linux 2.6.26-rc1 + +diff --git a/Makefile b/Makefile +index 5cf8258..4492984 100644 +--- a/Makefile ++++ b/Makefile +@@ -1,7 +1,7 @@ + VERSION = 2 + PATCHLEVEL = 6 +-SUBLEVEL = 25 +-EXTRAVERSION = ++SUBLEVEL = 26 ++EXTRAVERSION = -rc1 + NAME = Funky Weasel is Jiggy wit it + + # *DOCUMENTATION* +------------- + +And when we are finished we can use "git bisect reset" to go back to +the branch we were in before we started bisecting: + +------------- +$ git bisect reset +Checking out files: 100% (21549/21549), done. +Previous HEAD position was 2ddcca3... Linux 2.6.26-rc1 +Switched to branch 'master' +------------- + +Driving a bisection automatically +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The other way to drive the bisection process is to tell "git bisect" +to launch a script or command at each bisection step to know if the +current commit is "good" or "bad". To do that, we use the "git bisect +run" command. For example: + +------------- +$ git bisect start v2.6.27 v2.6.25 +Bisecting: 10928 revisions left to test after this (roughly 14 steps) +[2ec65f8b89ea003c27ff7723525a2ee335a2b393] x86: clean up using max_low_pfn on 32-bit +$ +$ git bisect run grep '^SUBLEVEL = 25' Makefile +running grep ^SUBLEVEL = 25 Makefile +Bisecting: 5480 revisions left to test after this (roughly 13 steps) +[66c0b394f08fd89236515c1c84485ea712a157be] KVM: kill file->f_count abuse in kvm +running grep ^SUBLEVEL = 25 Makefile +SUBLEVEL = 25 +Bisecting: 2740 revisions left to test after this (roughly 12 steps) +[671294719628f1671faefd4882764886f8ad08cb] V4L/DVB(7879): Adding cx18 Support for mxl5005s +... +... +running grep ^SUBLEVEL = 25 Makefile +Bisecting: 0 revisions left to test after this (roughly 0 steps) +[2ddcca36c8bcfa251724fe342c8327451988be0d] Linux 2.6.26-rc1 +running grep ^SUBLEVEL = 25 Makefile +2ddcca36c8bcfa251724fe342c8327451988be0d is the first bad commit +commit 2ddcca36c8bcfa251724fe342c8327451988be0d +Author: Linus Torvalds <torvalds@linux-foundation.org> +Date: Sat May 3 11:59:44 2008 -0700 + + Linux 2.6.26-rc1 + +:100644 100644 5cf8258195331a4dbdddff08b8d68642638eea57 4492984efc09ab72ff6219a7bc21fb6a957c4cd5 M Makefile +bisect run success +------------- + +In this example, we passed "grep '^SUBLEVEL = 25' Makefile" as +parameter to "git bisect run". This means that at each step, the grep +command we passed will be launched. And if it exits with code 0 (that +means success) then git bisect will mark the current state as +"good". If it exits with code 1 (or any code between 1 and 127 +included, except the special code 125), then the current state will be +marked as "bad". + +Exit code between 128 and 255 are special to "git bisect run". They +make it stop immediately the bisection process. This is useful for +example if the command passed takes too long to complete, because you +can kill it with a signal and it will stop the bisection process. + +It can also be useful in scripts passed to "git bisect run" to "exit +255" if some very abnormal situation is detected. + +Avoiding untestable commits +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Sometimes it happens that the current state cannot be tested, for +example if it does not compile because there was a bug preventing it +at that time. This is what the special exit code 125 is for. It tells +"git bisect run" that the current commit should be marked as +untestable and that another one should be chosen and checked out. + +If the bisection process is driven manually, you can use "git bisect +skip" to do the same thing. (In fact the special exit code 125 makes +"git bisect run" use "git bisect skip" in the background.) + +Or if you want more control, you can inspect the current state using +for example "git bisect visualize". It will launch gitk (or "git log" +if the DISPLAY environment variable is not set) to help you find a +better bisection point. + +Either way, if you have a string of untestable commits, it might +happen that the regression you are looking for has been introduced by +one of these untestable commits. In this case it's not possible to +tell for sure which commit introduced the regression. + +So if you used "git bisect skip" (or the run script exited with +special code 125) you could get a result like this: + +------------- +There are only 'skip'ped commits left to test. +The first bad commit could be any of: +15722f2fa328eaba97022898a305ffc8172db6b1 +78e86cf3e850bd755bb71831f42e200626fbd1e0 +e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace +070eab2303024706f2924822bfec8b9847e4ac1b +We cannot bisect more! +------------- + +Saving a log and replaying it +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If you want to show other people your bisection process, you can get a +log using for example: + +------------- +$ git bisect log > bisect_log.txt +------------- + +And it is possible to replay it using: + +------------- +$ git bisect replay bisect_log.txt +------------- + + +"git bisect" details +-------------------- + +Bisection algorithm +~~~~~~~~~~~~~~~~~~~ + +As the Git commits form a directed acyclic graph (DAG), finding the +best bisection commit to test at each step is not so simple. Anyway +Linus found and implemented a "truly stupid" algorithm, later improved +by Junio Hamano, that works quite well. + +So the algorithm used by "git bisect" to find the best bisection +commit when there are no skipped commits is the following: + +1) keep only the commits that: + +a) are ancestor of the "bad" commit (including the "bad" commit itself), +b) are not ancestor of a "good" commit (excluding the "good" commits). + +This means that we get rid of the uninteresting commits in the DAG. + +For example if we start with a graph like this: + +------------- +G-Y-G-W-W-W-X-X-X-X + \ / + W-W-B + / +Y---G-W---W + \ / \ +Y-Y X-X-X-X + +-> time goes this way -> +------------- + +where B is the "bad" commit, "G" are "good" commits and W, X, and Y +are other commits, we will get the following graph after this first +step: + +------------- +W-W-W + \ + W-W-B + / +W---W +------------- + +So only the W and B commits will be kept. Because commits X and Y will +have been removed by rules a) and b) respectively, and because commits +G are removed by rule b) too. + +Note for git users, that it is equivalent as keeping only the commit +given by: + +------------- +git rev-list BAD --not GOOD1 GOOD2... +------------- + +Also note that we don't require the commits that are kept to be +descendants of a "good" commit. So in the following example, commits W +and Z will be kept: + +------------- +G-W-W-W-B + / +Z-Z +------------- + +2) starting from the "good" ends of the graph, associate to each +commit the number of ancestors it has plus one + +For example with the following graph where H is the "bad" commit and A +and D are some parents of some "good" commits: + +------------- +A-B-C + \ + F-G-H + / +D---E +------------- + +this will give: + +------------- +1 2 3 +A-B-C + \6 7 8 + F-G-H +1 2/ +D---E +------------- + +3) associate to each commit: min(X, N - X) + +where X is the value associated to the commit in step 2) and N is the +total number of commits in the graph. + +In the above example we have N = 8, so this will give: + +------------- +1 2 3 +A-B-C + \2 1 0 + F-G-H +1 2/ +D---E +------------- + +4) the best bisection point is the commit with the highest associated +number + +So in the above example the best bisection point is commit C. + +5) note that some shortcuts are implemented to speed up the algorithm + +As we know N from the beginning, we know that min(X, N - X) can't be +greater than N/2. So during steps 2) and 3), if we would associate N/2 +to a commit, then we know this is the best bisection point. So in this +case we can just stop processing any other commit and return the +current commit. + +Bisection algorithm debugging +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +For any commit graph, you can see the number associated with each +commit using "git rev-list --bisect-all". + +For example, for the above graph, a command like: + +------------- +$ git rev-list --bisect-all BAD --not GOOD1 GOOD2 +------------- + +would output something like: + +------------- +e15b73ad3db9b48d7d1ade32f8cd23a751fe0ace (dist=3) +15722f2fa328eaba97022898a305ffc8172db6b1 (dist=2) +78e86cf3e850bd755bb71831f42e200626fbd1e0 (dist=2) +a1939d9a142de972094af4dde9a544e577ddef0e (dist=2) +070eab2303024706f2924822bfec8b9847e4ac1b (dist=1) +a3864d4f32a3bf5ed177ddef598490a08760b70d (dist=1) +a41baa717dd74f1180abf55e9341bc7a0bb9d556 (dist=1) +9e622a6dad403b71c40979743bb9d5be17b16bd6 (dist=0) +------------- + +Bisection algorithm discussed +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +First let's define "best bisection point". We will say that a commit X +is a best bisection point or a best bisection commit if knowing its +state ("good" or "bad") gives as much information as possible whether +the state of the commit happens to be "good" or "bad". + +This means that the best bisection commits are the commits where the +following function is maximum: + +------------- +f(X) = min(information_if_good(X), information_if_bad(X)) +------------- + +where information_if_good(X) is the information we get if X is good +and information_if_bad(X) is the information we get if X is bad. + +Now we will suppose that there is only one "first bad commit". This +means that all its descendants are "bad" and all the other commits are +"good". And we will suppose that all commits have an equal probability +of being good or bad, or of being the first bad commit, so knowing the +state of c commits gives always the same amount of information +wherever these c commits are on the graph and whatever c is. (So we +suppose that these commits being for example on a branch or near a +good or a bad commit does not give more or less information). + +Let's also suppose that we have a cleaned up graph like one after step +1) in the bisection algorithm above. This means that we can measure +the information we get in terms of number of commit we can remove from +the graph.. + +And let's take a commit X in the graph. + +If X is found to be "good", then we know that its ancestors are all +"good", so we want to say that: + +------------- +information_if_good(X) = number_of_ancestors(X) (TRUE) +------------- + +And this is true because at step 1) b) we remove the ancestors of the +"good" commits. + +If X is found to be "bad", then we know that its descendants are all +"bad", so we want to say that: + +------------- +information_if_bad(X) = number_of_descendants(X) (WRONG) +------------- + +But this is wrong because at step 1) a) we keep only the ancestors of +the bad commit. So we get more information when a commit is marked as +"bad", because we also know that the ancestors of the previous "bad" +commit that are not ancestors of the new "bad" commit are not the +first bad commit. We don't know if they are good or bad, but we know +that they are not the first bad commit because they are not ancestor +of the new "bad" commit. + +So when a commit is marked as "bad" we know we can remove all the +commits in the graph except those that are ancestors of the new "bad" +commit. This means that: + +------------- +information_if_bad(X) = N - number_of_ancestors(X) (TRUE) +------------- + +where N is the number of commits in the (cleaned up) graph. + +So in the end this means that to find the best bisection commits we +should maximize the function: + +------------- +f(X) = min(number_of_ancestors(X), N - number_of_ancestors(X)) +------------- + +And this is nice because at step 2) we compute number_of_ancestors(X) +and so at step 3) we compute f(X). + +Let's take the following graph as an example: + +------------- + G-H-I-J + / \ +A-B-C-D-E-F O + \ / + K-L-M-N +------------- + +If we compute the following non optimal function on it: + +------------- +g(X) = min(number_of_ancestors(X), number_of_descendants(X)) +------------- + +we get: + +------------- + 4 3 2 1 + G-H-I-J +1 2 3 4 5 6/ \0 +A-B-C-D-E-F O + \ / + K-L-M-N + 4 3 2 1 +------------- + +but with the algorithm used by git bisect we get: + +------------- + 7 7 6 5 + G-H-I-J +1 2 3 4 5 6/ \0 +A-B-C-D-E-F O + \ / + K-L-M-N + 7 7 6 5 +------------- + +So we chose G, H, K or L as the best bisection point, which is better +than F. Because if for example L is bad, then we will know not only +that L, M and N are bad but also that G, H, I and J are not the first +bad commit (since we suppose that there is only one first bad commit +and it must be an ancestor of L). + +So the current algorithm seems to be the best possible given what we +initially supposed. + +Skip algorithm +~~~~~~~~~~~~~~ + +When some commits have been skipped (using "git bisect skip"), then +the bisection algorithm is the same for step 1) to 3). But then we use +roughly the following steps: + +6) sort the commit by decreasing associated value + +7) if the first commit has not been skipped, we can return it and stop +here + +8) otherwise filter out all the skipped commits in the sorted list + +9) use a pseudo random number generator (PRNG) to generate a random +number between 0 and 1 + +10) multiply this random number with its square root to bias it toward +0 + +11) multiply the result by the number of commits in the filtered list +to get an index into this list + +12) return the commit at the computed index + +Skip algorithm discussed +~~~~~~~~~~~~~~~~~~~~~~~~ + +After step 7) (in the skip algorithm), we could check if the second +commit has been skipped and return it if it is not the case. And in +fact that was the algorithm we used from when "git bisect skip" was +developed in git version 1.5.4 (released on February 1st 2008) until +git version 1.6.4 (released July 29th 2009). + +But Ingo Molnar and H. Peter Anvin (another well known linux kernel +developer) both complained that sometimes the best bisection points +all happened to be in an area where all the commits are +untestable. And in this case the user was asked to test many +untestable commits, which could be very inefficient. + +Indeed untestable commits are often untestable because a breakage was +introduced at one time, and that breakage was fixed only after many +other commits were introduced. + +This breakage is of course most of the time unrelated to the breakage +we are trying to locate in the commit graph. But it prevents us to +know if the interesting "bad behavior" is present or not. + +So it is a fact that commits near an untestable commit have a high +probability of being untestable themselves. And the best bisection +commits are often found together too (due to the bisection algorithm). + +This is why it is a bad idea to just chose the next best unskipped +bisection commit when the first one has been skipped. + +We found that most commits on the graph may give quite a lot of +information when they are tested. And the commits that will not on +average give a lot of information are the one near the good and bad +commits. + +So using a PRNG with a bias to favor commits away from the good and +bad commits looked like a good choice. + +One obvious improvement to this algorithm would be to look for a +commit that has an associated value near the one of the best bisection +commit, and that is on another branch, before using the PRNG. Because +if such a commit exists, then it is not very likely to be untestable +too, so it will probably give more information than a nearly randomly +chosen one. +</ |