'Building the extractor
@param content_type: content_type of the part for which the extractor is defined
@param text_elements: default text elements. See self.addTextElement(...)'

def __init__(self, content_type, *text_elements, **kwargs):

    self.content_type = content_type
    self.text_elts_xpaths = [etree.XPath(('//' + te), namespaces=ns_map) for te in text_elements]
    if ('separator' in kwargs):
        self.separator = kwargs['separator']
    else:
        self.separator = ''
    return
