def filter_fn(filename):
    """Filter interesting coverage files.

        >>> filter_fn('z3c.coverage.__init__.cover')
        True
        >>> filter_fn('z3c.coverage.tests.cover')
        False
        >>> filter_fn('z3c.coverage.tests.test_foo.cover')
        False
        >>> filter_fn('z3c.coverage.ftests.test_bar.cover')
        False
        >>> filter_fn('z3c.coverage.testing.cover')
        True
        >>> filter_fn('z3c.coverage.testname.cover')
        True
        >>> filter_fn('something-unrelated.txt')
        False
        >>> filter_fn('<doctest something-useless.cover')
        False

    """
    parts = filename.split('.')
    return (filename.endswith('.cover') and
            not filename.startswith('<') and
            'tests' not in parts and
            'ftests' not in parts)