hi, i just wanted to ask when is "" inside "-quoted string parsed to simple "?
(like " ""a"" " is same as ' "a" ')
is this done by preprocessor (like # and `)?
btw. is this behavior even documented somewhere?

Yeah, it seems I forgot to document it - as also some other similar features that are so "standard" that it's easy to forget about documenting them.
And this conversion is done by the first stage of preprocessor, the "convert_line" routine.