function write_file(filename)
//
// Create a file on disk, and then ask the user for data.
//   Copy everything the user types into the file.
//   Quit when the user types the Return key (an empty line),
//   signalling no more data.
//   
// Arguments:
//      
//         filename          (input)  name of disk file to create
//                   

  // first, we try to open the file for writing (which will
  //   create it if it doesn't exist yet)
  fid = mopen(filename, 'w');
  if (fid == -1) 
    error('write_file: cannot open file for writing');
  end

  done_yet = 0;
  while (done_yet == 0)

    // we use the optional second argument 's' to the INPUT function,
    //   which means that everything the user types is kept together
    //   as a string, instead of being broken up into an array of values.
    user_string = input('gimme stuff for the file (blank line to quit) ','s');

    // if the user's string is equalt to a single space,
    //    then the user typed nothing but a carriage return, so quit
    if (user_string == " ") then
      done_yet = 1;
    else
      mfprintf(fid, '%s\n', user_string);
    end
    
  end

  // remember to close the file before we return;
  //   otherwise, some of the material might not be written onto disk
  mclose(fid);

endfunction