Right Labib :) For other readers who might be interested, here is a reference about distribution of negation over a logic statement, which states:

> !(A && B) = (!A || !B)

So when I tried to remove the extra negation, I should have changed the AND operator to an OR operator. But this alone wouldn't fix the "minor version" problem, the full fix is already  stated in comment #6.

Labib, I am looking forward to seeing your changes merged into the master branch after Mohammad's approval, and your name added to the gnuastro manual's developers list :)

Best wishes :)

Oops, I found out the condition has a logical bug. For example, version 8.10 wouldn't pass the test correctly, since ver[1] < 12 and there's an && in the condition. To make it right, I suggest this alternative:

if( (ver[0]<7) || (ver[0]==7 && ver[1]<12) ) exit(1);

So now, if the major version number is less than 7, WCSLIB is definitely old. But, if it the major version is 7 AND the minor is less than 12, again, it is old. It is considered as recent otherwise.

Did I get it right Labib?

Nicely done Labib! The message is clear and concise, and the variables and checks are put in good places. Well done. You should go for the PR :) Mohammad will also take a look at it before merging to master and setting this task as complete.

I only have a question about lines 563-565:

int ver[3]={-1,-1,-1};
wcslib_version(ver);
if(!(ver[0]>=7 && ver[1]>=12))exit(1);

Here, we're declaring `ver` of size 3. Does WCS follow the x.x.x versioning pattern? Or is it just there to follow some kind of a  standard that the `wcslib_version` function is following?

Hello there Labib,

Thank you for showing interest in completing this task.

Indeed, we can check the WCS version inside 'configure.ac'. The OPTIONAL
warnings echo the following message:

> "OPTIONAL warnings/dependencies (for improved functionality):"

So I'd say since having an 'old' WCS does NOT BREAK the entire program for
every use case, this could be categorized under 'improved functionality'.

About getting the version, maybe we could somehow mimic the
"$gsl_version_old" variable in 'configure.ac'. In case the version is lower
than 7.12, a new variable like "$wcs_version_old" could be set to "yes" and
then a warning could be printed under the OPTIONAL warnings section
informing the user that "warp" might crash in some scenarios (as explained
in the gnuastro manual).

Sepideh and Mohammad have already let Mark Callabreta know that WCSLIB 7.11 fails when calculating specific coordinates. He has fixed it in WCSLIB 7.12, and in the changelog pointed out:

>  - In wcsp2s() and wcss2p(), fixed handling of status returns from
>    linp2x() and linx2p() relating to distortion functions, specifically
>    with respect to setting the stat[] vector.  Reported by Sepideh
>    Eskandarlou (via Mohammad Akhlaghi).

That is why we have put a warning in Warp's documentation about this and encouraged users to install at least WCSLIB 7.12. However, some might simply miss this important warning. Hence, we should check user's WCSLIB version while running warp, and print a warning to prevent bad results.