Atom-Or-Comma-List names a function, with lambda list (VALUE):
Return VALUE, possibly by turning it into a comma-delimited string.
An ATOM VALUE is returned intact.
A one-member sequence is returned as the first element of the sequence.
Anything else should be a list that will be turned into a comma-delimited string.
Used in generating HTTP headers.
Defined in file src/web.lisp.