[Vala] Testing generic parameters for numeric type? For method implementation?

[Charles Hixson <charleshixsn earthlink net>]

How should one test a generic parameter to determine whether or not it 
is of a numeric type?
Should I just go through a long If tree matching it's type name against 
all of the predefined numeric types?

Is there some way to determine whether a generic parameter responds to 
to_string()?  At run time?  (The compiler seems to be very unhappy with 
attempts to use that unless you cast the parameter to an appropriate type.)