@classmethod
def _find_child_from_identifier(cls, anchor, identifier):
    """
        Find a child of anchor that matches the specified identifier.

        Parameters
        ----------
        anchor : xml.etree.ElementTree.Element
        identifier : unicode
            Name of the element (may be indexed).

        Returns
        -------
        child : None or xml.etree.ElementTree.Element
            The first child that matches the identifier or
            ``None`` if no child matches the identifier.

        Examples
        --------
        >>> import xml.etree.ElementTree as xml
        >>> root = xml.Element('Root')
        >>> root.append(xml.Element('A'))
        >>> root.append(xml.Element('A'))
        >>> XMLAdaptor._find_child_from_identifier(root, 'A')
        <Element 'A' at 0x7f3212b44b10>
        >>> XMLAdaptor._find_child_from_identifier(root, 'A[0]')
        <Element 'A' at 0x7f3212b44b10>
        >>> XMLAdaptor._find_child_from_identifier(root, 'A[1]')
        <Element 'A' at 0x7f32208924d0>
        >>> XMLAdaptor._find_child_from_identifier(root, 'A[1]').append(xml.Element('B'))
        >>> XMLAdaptor._find_child_from_identifier(root, 'B')  # => None
        """
    index = cls._parse_index_from_element_identifier(identifier)
    if index is not None:
        try:
            return anchor.findall(cls._parse_name_from_element_identifier(
                identifier))[index]
        except IndexError:
            return None
    return anchor.find(identifier)
