It is very useful.
But it seems to be a little difficult to make it perfectly in the
In the case of standard format of GTAGS (by gtags without -c option),
it can be realized by just rewriting parsers.
In the case of compact format (by gtags -c), it is difficult.
In the present implementation, GTAGS has no source code and global(1)
has no knowledge about languages. So, global(1) need to read source
code and modify definition lines.
There may be a function limited to the standard format.