What is the beef with \longa stems?

[David Kastrup]

I am trying to fiddle around with some code for stems on notes of
unequal duration, and I noticed that \longa notes are for some reason
not handled by the normal stem routines.

Why?

For example, the following falls apart:

{ \time 4/1 <g'' c' e'>\longa }

whereas the same will look fine with half notes instead of longas.  So
why are longas treated specially?

Thanks

-- 
David Kastrup