Troubleshooting / Bugs: Snippet mirroring

The “once” snippet for C++ appears to be broken. But upon inspection, it appears to be the way that intype deals with this particular case. The snippet is:
{ title: '#ifndef (Guard)' tab_trigger: 'once' scope: 'source.c, source.c++' content: '#ifndef __${1:FILENAME_HPP}__ #define __$1 __ $0 #endif' }
The problem here is that where it’s supposed to mirror it you just get:
#define __
and nothing else. If you add spaces in the snippet around the second $1 then it works as expected, but with the spaces obviously.

OK, as I was writing that I had an idea – “is there a particular space which makes it work, or both?”. I tried it with adding a space just before $1 and just after, and the results are:
#define __ $1__ // Doesn't work #define __$1 __ // Does work
So I guess the problem is that the snippet mirror code doesn’t realise that _ isn’t a number. (It doesn’t like letters directly after also.)

Edit: as a temporary fix, I can have the second instance as
#define __${1}__

On the other side, using a double underscore is not recommended.
Here a modified version of this snippet:

{ title: '#ifndef (Guard)' tab_trigger: 'once' scope: 'source.c, source.c++' content: '#ifndef ${1:__}${2:FILENAME_HPP}$1 #define $1$2$1 $0 #endif' }