Dear Categorists - Given a category C with pullbacks we can define a bicategory Span(C) where objects are objects of C, morphisms are spans - composed using pullback - and 2-morphisms are maps between spans. Have people tried to categorify this yet? Suppose we have a 2-category C with pseudo-pullbacks. Then we should be able to define a tricategory Span(C). Has someone done this? Or maybe people have gotten some partial results, e.g. in the case where C = Cat. I'd like to know about these! Best, jb