Interface for controlling fermata

[Jim Long]

What is the appropriate grob name for a fermata?

I want to:

      \once \override Fermata #'Y-offset = #2

but Lily tells me that's not correct:

fermata-test.ly:18:12: warning: not a grob name, `Fermata'

Looking at

http://lilypond.org/doc/v2.16/Documentation/internals/all-layout-objects

and

http://lilypond.org/doc/v2.16/Documentation/internals/graphical-object-interfaces

I'm still not finding the interface I need.

Thank you,

Jim