Parse the comma separated header values for which each extended token may have associated parameters. Empty tokens are allowed and are ignored. The tokens are canonicalised to lower case and returned in a list and in the same order that they appear in the headers. The parameters in each extended token are parsed and they are returned as a list of the token followed by an alist of attribute and value pairs.
- ((token-1 (attrib-1a . value-1a) (attrib-1b . value-1b) ...)
- (token-2 (...) (...) (...)) ...)