code: def create_class_instance(element, element_id, doc_id):
      
      xsi_type = get_xsi_type(element)
      element_class = XSI_TYPE_CLASSES[xsi_type]
      return element_class.from_etree(element)
docs: given an Salt XML element, returns a corresponding `SaltElement` class
      instance, i.e. a SaltXML `SToken` node will be converted into a
      `TokenNode`.
  
      Parameters
      ----------
      element : lxml.etree._Element
          an `etree._Element` is the XML representation of a Salt element,
          e.g. a single 'nodes' or 'edges' element
      element_id : int
          the index of element (used to connect edges to nodes)
      doc_id : str
          the ID of the SaltXML document
  
      Returns
      -------
      salt_element : SaltElement
          an instance of a `SaltElement` subclass instance, e.g. a `TokenNode`,
          `TextualRelation` or `SaltLayer`