Create a file with a single line of code:

def x(arg): return reduce(sum, [])

On Python 3.1.1 (I used Windows 64-bit), run 2to3 on it returns the
following error:

PS C:\Users\jaraco\projects\jaraco.util\jaraco\util>
C:\python\Tools\Scripts\2to3.py .\functools.py
RefactoringTool: Skipping implicit fixer: buffer
RefactoringTool: Skipping implicit fixer: idioms
RefactoringTool: Skipping implicit fixer: set_literal
RefactoringTool: Skipping implicit fixer: ws_comma
Traceback (most recent call last):
  File "C:\python\Tools\Scripts\2to3.py", line 6, in <module>
    sys.exit(main("lib2to3.fixes"))
  File "C:\Python\lib\lib2to3\main.py", line 159, in main
    options.processes)
  File "C:\Python\lib\lib2to3\refactor.py", line 616, in refactor
    items, write, doctests_only)
  File "C:\Python\lib\lib2to3\refactor.py", line 276, in refactor
    self.refactor_file(dir_or_file, write, doctests_only)
  File "C:\Python\lib\lib2to3\refactor.py", line 656, in refactor_file
    *args, **kwargs)
  File "C:\Python\lib\lib2to3\refactor.py", line 328, in refactor_file
    tree = self.refactor_string(input, filename)
  File "C:\Python\lib\lib2to3\refactor.py", line 358, in refactor_string
    self.refactor_tree(tree, name)
  File "C:\Python\lib\lib2to3\refactor.py", line 392, in refactor_tree
    self.traverse_by(self.post_order_heads, tree.post_order())
  File "C:\Python\lib\lib2to3\refactor.py", line 416, in traverse_by
    new = fixer.transform(node, results)
  File "C:\Python\lib\lib2to3\fixes\fix_reduce.py", line 33, in transform
    touch_import('functools', 'reduce', node)
  File "C:\Python\lib\lib2to3\fixer_util.py", line 289, in touch_import
    root = find_root(node)
  File "C:\Python\lib\lib2to3\fixer_util.py", line 265, in find_root
    assert node.parent, "Tree is insane! root found before "\
AssertionError: Tree is insane! root found before file_input node was found.

2to3 on Python 2.6.4 does not exhibit the same error.

Remove 'arg' or the call to reduce and the problem is not exhibited.

Fixed in r76061.