How many parameters does an elisp function take?

[Alan Mackenzie]

Is it possible to determine at run time how many parameters an elisp
function takes?  For example, I'd like to write something like:

(how-many-params 'null)

and have it evaluate to 1.  Or something like that.  Together with some
reasonable convention for indicating &optional and &rest arguments.

-- 
Alan Mackenzie (Munich, Germany)
Email: aacm@muuc.dee; to decode, wherever there is a repeated letter
(like "aa"), remove half of them (leaving, say, "a").