/a vs. /one

[Will Oram]

I know either HW or Jan called this a bug two versions ago, but it's a 
problem I can't quite pinpoint.

I have a definitions file for storing frequently used markups. This 
line causes all instances of it in music (eg, a4_\a ) to be called a 
parse error:

a = \markup { \bold {a 2} }

The line below it, though, is accepted everywhere:

one     = \markup { \bold {1.}}

What's the difference between these two? How can I tweak it?

Best,

Will Oram // Genius @ Large // AIM spamguy21
spamguy (at) foxchange (dot) com // wro1 (at) cwru (dot) edu