> Strictly speaking, in PostScript also each page can be a > different size (though there is nothing built-in to groff to > arrange this).
This is correct, but because it is not clear what it would mean to include a multi-page document as a graphic, let's simply agree it should not be done. For a single-page document, BoundingBox and PageBoundingBox (of the single page) should be equal. And the encapsulated Postscript (EPS) specification only allows a single page and only mentions a BoundingBox comment. (EPS is normal Postscript where a few (mainly device-control) operators are not allowed.) EPS is the preferred format for including graphics, but I believe we should be as lenient as possible, since most programs do output reasonable Postscript that can be included, even if it is not called ".eps".
