programming error: Stem dir must be up or down.

[Kieren MacMillan]

Hi all,

In 2.13.16, I'm getting a

programming error: Stem dir must be up or down.
continuing, cross fingers

and then Lilypond dies.

Unfortunately, I haven't been able to make a minimal example, but I was hoping 
someone -- like the original programmer who wrote the error code -- could give 
me a hint about what might be causing the problem so I can hunt it down.

Thanks,
Kieren.