Can't refer to variable directly after defining it

[caagr98]

For some reason, it seems I can't refer to a variable directly after 
defining it. As soon as I do /anything/ else, it works, but `foo={...} 
\foo` gives errors.

```
$ cat bug.ly
\version "2.18.2"
foo = {c' c' c' c'}
% bar = {d' d' d' d'}
% {e' e' e' e'}
% #foo
\foo
$ lilypond bug.ly
GNU LilyPond 2.18.2
Processing `bug.ly'
Parsing...
bug.ly:6:1: error: unknown escaped string: `\foo'

\foo
bug.ly:6:5: error: syntax error, unexpected end of input
\foo

fatal error: failed files: "bug"
```

Uncommenting either of the three commented-out lines makes it work, all 
with the same resulting PDF.

I'm currently using `\void {}` as a workaround, but I'd obviously prefer 
not having to do that.