Variable argument type to music function

[Carl D. Sorensen]

I'm trying to allow predefine fret diagrams to use the verbose style as well
as the terse style.

I'd like to pass a diagram-definition to a music function that can be either
a list or a string.  If it's a list, I'll treat the definition as a verbose
definition.  If it's a string, I'll treat it as a terse definition.  I've
got everything in place except for the data type check.

What kind of a type check can I use to see if a variable is a list (or pair)
or a string?

I'm after something like:

(define-music-function (parser location key-symbol tuning shape-definition)
   (symbol? pair? ????)
 
Where ??? would be the true for either a pair or a string.

Thanks,

Carl