Contextual 'iskeyword'?

[Tim Chase]

In some text, I've got compound words separated by a single 
hyphen.  For convenience of yanking, I've added the hyphen to my 
iskeyword setting which works nicely for the most part.  However, 
I also use a doubled-hyphen to the effect one would use an 
em-dash which leads to the unwanted situation that a yank of a 
"word" now includes the first word of the subordinate sentence 
structure--such as this where the dashes are doubled--and effects 
my ^N/^P searching (as duplicates appear for entries followed by 
the double-dash).

I'm on the prowl for some way to keep the iskeyword behavior for 
things like "doubled-hyphen" and "em-dash" in the above 
paragraph, but exclude things like "structure--such" and 
"doubled--and", limiting the "word" to things with a dash only if 
that dash is not repeated.  Something like "\w-\w" but not 
"\w-\+\w" (assuming that "-" isn't part of iskeyword for this 
example)

Any hints?

Thanks,

-tim