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extra lambda list keywords

I just noticed the bit on p. 65 of the manual where it says:

"Implementations of Common Lisp are free to provide additional lambda-list

Is this *really* useful to any implementation?  Although you can supposedly
find out about the additional keywords from the lambda-list-keywords constant,
allowing implementation-specific crud to appear in lambda lists seems like
another good way to frustrate would-be implementors of portable code walkers.
In particular, I'm concerned that if implementations try to extend the syntax
of what can appear in lambda lists, code walkers would have a very hard time
identifying default values and other bits of evaluable code in the lambda
list.  Not to mention that a routine that tries to parse lambda lists could
get very confused if, for example, it found a strange keyword &foo sitting
where it expected to find only an &rest or &key.