Reverse chaining substitutions do not support glyph deletion

I just tried to compile a .glyphs file with some `rsub` rules. The adobe spec explicitly states:

> As with substitute, if the replacement glyph is the reserved word NULL then the reverse substitution has no replacement, removing the glyph from the sequence. Omitting the by clause is equivalent to adding by NULL.

but I get

> Compiling UFO failed: :10:1: Reverse chaining substitutions do not support glyph deletion

Is there a reason (other nobody used it yet) why this shouldn’t be implemented?

This is the code:
```
rsub [D E] C' by NULL;
```

I tried with makeOTF and it doesn’t like the code, either.

here a file for testing:
[ReverseChainSingleSubst.glyphs.zip](https://github.com/user-attachments/files/24838298/ReverseChainSingleSubst.glyphs.zip)