Suggestion on display of invisible characters

[Harry Putnam]

A useful inhancement of emacs display tools would be to provide a way
to *easily* display normally invisible characters such as tabs and
newlines.

Something along the lines of what vi does with the `:list' command.

An example looks like this in normal emacs display:

        line
                line  
line
line    

But viewed with vi (acutally vim) `:list' command, it looks like: 

^Iline  $
^I^Iline$
line   $
line$
$

The tabs appear as ^I and the newlines are clearly marked out with `$'.
It might be nicer if the tabs were marked but still used the same
amount of space.

This can be very handy at times.  Like when having a problem created
by trailing white space or when moving files between platforms and
editors where tabs and space may be a factor.

I understand the coming emacs-21 will have a feature to display
trailing white space in a differnt face which will be vrey handy.

I'm told, `invisible characters can be displayed by monkeying around
with `display tables', but what I'm talking about here is a toggle type
command that quickly and easily displays invisible stuff but also
allows editing.

Tabs and newlines seem the most usefull but there may be other `invisible'
characters that would be usefull too.