code: def get_or_add_childTnLst(self):
          
          childTnLst = self._childTnLst
          if childTnLst is None:
              childTnLst = self._add_childTnLst()
          return childTnLst
docs: Return parent element for a new `p:video` child element.
  
          The `p:video` element causes play controls to appear under a video
          shape (pic shape containing video). There can be more than one video
          shape on a slide, which causes the precondition to vary. It needs to
          handle the case when there is no `p:sld/p:timing` element and when
          that element already exists. If the case isn't simple, it just nukes
          what's there and adds a fresh one. This could theoretically remove
          desired existing timing information, but there isn't any evidence
          available to me one way or the other, so I've taken the simple
          approach.