how to prohibit insertion at beginning of textbuffer?

[george young <gry ll mit edu>]

[pygtk-1.99.16, gtk+-2.2.1, python 2.3a1, x86 linux]

I have a textbuffer with a textview.  The program inserts some text;
now I want the user to only be able to append to the buffer, not alter
the existing text, *or* insert before the existing text.  I insert the
text readonly with a tag having 'editable' attribute false,
but how can I prohibit insertion at the beginning of the buffer?  
It looks like I could anchor a mark there, but I don't see how to tie
the mark and the tagged text...

-- George Young

-- 
 I cannot think why the whole bed of the ocean is
 not one solid mass of oysters, so prolific they seem. Ah,
 I am wandering! Strange how the brain controls the brain!
        -- Sherlock Holmes in "The Dying Detective"