not only is it undocumented, it is more than likely unintended too. as tempting as it can be to showcase one's knowledge of little known esoteric parser quirks, we must remember that there is a time and a place for such things :P it's not advisable to demonstrate usage of those quirks without further explanation and a word of caution.

it is not easy to infer, even with context, the meaning of $(,code), the help file does not explain its tendency to evaluate twice when used that way, and it's something that could very likely be fixed in a later version.

once upon a time it was discovered that $(,,code) evaluated 'code' and returned an overall result of $null. this was before /noop and $regsubex() existed, /.echo -q was considered longer and more cumbersome, and $(,, ) appeared to provide a convenient way to use identifiers like $regsub() and $findfile() inline while discarding their result. and it was until, without warning, it was modified to behave similar to $(, code). this broke quite a few scripts and served as a warning: do not get in the habit of using undocumented features, especially if perfectly documented and more readable alternatives exist!