If the optional filename argument is not specified, the
file requester is opened, and you are prompted to select a file (you
can inhibit the file requester opening by using the
command; see NoFileReq).
If you escape from the file requester, you can input the file name on the command line, the default being the current document name, if available.
If at the time the command is issued the current document is marked as modified, you have to confirm the action.