How do I determine if a function is defined?

[Bruce Korb]

Hi,

I keep trying obvious things, like:

  (if (function? mumble) (mumble))

and get:

  ERROR: Unbound variable: mumble

and then it dies.  I just want to do one thing if it is
a function and another if it is either undefined or even
anything else.  How to do that seems completely non-obvious.
Thank you.