[texhax] length question

[Tom Sgouros]

Hello again:

I have a book I'm laying out that has a significant margin for
marginalia like figures and notes.  It alternates sides to be in the
outer margin.

even pages:
|<-\marginparwidth -><\marginparsep><------\textwidth-------->|
odd pages:
|<------\textwidth--------><\marginparsep><-\marginparwidth ->|

Is there some way to arrange rubber lengths so that I can have a \left
and a \right so that \left\unhbox0\right will be inside the text column
if \wd0 is less than \textwidth, but will intrude on the marginpar side
if it's greater?  

Failing that, how about centered on the whole page?  

I've been unable to think of a way to do either of these, but TeX has
been good for my humility, so I'm asking.

I've done most of what I need with \checkodd and \ifoddpage (using the
memoir class) but that won't work for longtable, and I wondered if
there was a cleaner way to accomplish this than all the \if statements
I've been using.

Many thanks,

 -tom

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