Looks up among the currently open document the one whose filename is same as the given URL and returns the corresponding document node. If no document's filename matches exactly the given URL, then several heuristic matches are tried: if the URI is a relative filename, it is tilde-expanded and resolved (using the current working directory as a base) and the lookup is restarted with the absolute filename; finally, a lookup identifying filenames with URLs of the file:// protocol is attempted. If the lookup fails completely, an empty node set is returned.