Allow testing if we are inside a \csname .. \endcsname construct as there a number of operations are not allowed.

This is actually one area of TeX where you really can't provided a programmed alternative that would still have acceptable running time. So having such a primitive would be more than useful. Thanh kindly programmed it for us while we talked about it.

Recent comment by Morten:

I gave stuff like \ifincsname some thought a couple of weeks ago and discovered that what Thanh implemented that day isn't safe enough. What if someone does \csname xxx \csname yyy\endcsname zzz\endcsname? Then the second \csname will set the switch false. So we must add a temporary boolean to record the value of if_in_csname when we enter the function and then set if_in_csname to that when we exit.  Plus we also have to do something similar for if_in_csname (\ifcsname in eTeX).

Reply by Thanh:

the new primitive \ifincsname is included in the alpha version of pdftex http://sarovar.org/projects/pdftex/

I will have a look at your patch and submit it.