Is there a way to get the parent of an element, i.e. the element in which an element is in?

Use cases: when printing the traversal of the elements (show_level) or when checking whether a Str is in a ListItem (str_in_listitem). See my python example below.

If there is no parent in Lua, how are those use cases implemented in a Lua filter?

from panflute import *
from panflute.utils import debug

def get_level(elem: Element, level=0):
    if elem.parent is None:
        return level
    else:
        return get_level(elem.parent, level=level + 1)

def is_ancestor_of(elem: Element, clazz: Element):
    if elem.parent is not None:
        if type(elem.parent) == clazz:
            return True
        else:
            return is_ancestor_of(elem.parent, clazz)
    else:
        return False

def show_level(elem: Element, doc: Doc):
    debug("  " * get_level(elem) + elem.tag)

def str_in_listitem(elem: Element, doc: Doc):
    if type(elem) == Str:
        if is_ancestor_of(elem, ListItem):
            debug(stringify(elem))

if __name__ == "__main__":
    run_filters([show_level, str_in_listitem])

Appying this filter to the following input:

[ Para [ Str "some" , Space , Str "text" ]
, BulletList
    [ [ Plain [ Str "item1" ] ]
    , [ Plain [ Str "item2" ]
      , BulletList
          [ [ Plain [ Str "subitemA" ] ]
          , [ Plain [ Str "subitemB" ] ]
          ]
      ]
    ]
]

shows the following output for show_level:

  MetaMap
    Str
    Space
    Str
  Para
        Str
      Plain
    ListItem
        Str
      Plain
            Str
          Plain
        ListItem
            Str
          Plain
        ListItem
      BulletList
    ListItem
  BulletList
Doc

and the following for str_in_listitem:

item1
item2
subitemA
subitemB