newbie question about "unaccent" symbol

[Jacob Smullyan]

Hi --

I'd like to mark a note as unaccented using the symbol whose use I
associate with Schoenberg, the bottom half of a flattened circle
(there must be a name for this symbol, but I don't know what it is).
How can this be done?

I'm new to lilypond, and haven't entirely figured out how to navigate
through the internals documentation, so perhaps I've missed this.  

I'm currently using version 1.6.6.

Thanks!

Jacob Smullyan

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