lxml convert element to elementtree

The following test code reads a file, and using lxml.html generates the leaf nodes of the DOM/Graph for the page.

However, I'm also trying to figure out how to get the input from a "string". Using:

lxml.html.fromstring(s)

doesn't work, as this generates an Element as opposed to an ElementTree.

So, I'm trying to figure out how to convert an element to an ElementTree.

[my test code]

import lxml.html
from lxml import etree    # trying this to see if needed 
                          # to convert from element to elementtree


  #cmd='cat osu_test.txt'
  cmd='cat o2.txt'
  proc=subprocess.Popen(cmd, shell=True,stdout=subprocess.PIPE)
  s=proc.communicate()[0].strip()

  # s contains HTML not XML text
  #doc = lxml.html.parse(s)
  doc = lxml.html.parse('osu_test.txt')
  doc1 = lxml.html.fromstring(s)

  for node in doc.iter():
  if len(node) == 0:
     print "aaa ",node.tag, doc.getpath(node)
     #print "aaa ",node.tag

  nt = etree.ElementTree(doc1)        <<<<< doesn't work.. so what will??
  for node in nt.iter():
  if len(node) == 0:
     print "aaa ",node.tag, doc.getpath(node)
     #print "aaa ",node.tag

UPDATE 1:

(parsing html instead of xml) Added the changes suggested by Abbas. got the following errs:

    doc1 = etree.fromstring(s)
  File "lxml.etree.pyx", line 2532, in lxml.etree.fromstring (src/lxml/lxml.etree.c:48621)
  File "parser.pxi", line 1545, in lxml.etree._parseMemoryDocument (src/lxml/lxml.etree.c:72232)
  File "parser.pxi", line 1424, in lxml.etree._parseDoc (src/lxml/lxml.etree.c:71093)
  File "parser.pxi", line 938, in lxml.etree._BaseParser._parseDoc (src/lxml/lxml.etree.c:67862)
  File "parser.pxi", line 539, in lxml.etree._ParserContext._handleParseResultDoc