The key words "CAR", "CDR", "XARN", "ANUS", "ANII",
"ANAE", "BJARNE", "SEPPLES", "PYTHONG", and "[spoiler]PRAGUE[/spoiler]" in this
document are to be interpreted as described in [RFC2119].
there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior involving a "car"
there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing to use a "cdr"
"xarn" - the definition is an absolute prohibition of the specification
"an*" - the definition is an absolute requirement of the specification
"bjarne" is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels ANUS. it enhances the product while another vendor may omit the same ANUS. An implementation which does not include a particular ANUS MUST be prepared to interoperate with another ANUS which does include the option, though perhaps with reduced satori. In the same ANUS an implementation which does include a particular ANUS MUST be prepared to interoperate with another ANUS which does not include the ANUS (except, of course, for the feature the ANUS provides.)