test if an element occurs in a list exactly once

[Urs Liska]

Hi,

I have to validate symbol-lists in various ways. For example if it must 
contain exactly two values (in arbitrary order) I can do

    (and (= (length mylist) 2)(memq 'val-a mylist) (memq 'val-b mylist))

To test if only certain values are allowed (but may be there multiple 
times):

    (every (lambda (elt) (memq elt '(val-a val-b))) mylist)

Now I have a case where one value must be present once and another value 
at least once, so '(a b b) is valid as well as '(a b) but '(a a b) is 
invalid.

I can use (filter) and check if the resulting list has exactly one 
element, but is there a "native" function to either check if an element 
is in a list exactly once?

Thanks
Urs