It would seem almost inevitable, yet still hard to fathom: rival gang members, angered and perhaps aggrieved over some recent disagreement, seek to continue or end the dispute the next time they meet — even if that encounter occurs at a hospital. This happened on Wednesday, in a waiting area at Bronx-Lebanon Hospital Center's emergency room. A man being treated for facial injuries bumped into a rival gang member who had also shown up at the hospital, and both of them called for backup crews who came to the hospital that evening. One of those who came to the hospital brought a gun and fired it, wounding a hospital nurse and a security guard. But even though hospital emergency rooms often sit at the intersection of mayhem and medicine, they tend to be fire-free zones to New York's criminal class.