Append `a:p` elements taken from *spanned_tc*.

        Any non-empty paragraph elements in *spanned_tc* are removed and
        appended to the text-frame of this cell. If *spanned_tc* is left with
        no content after this process, a single empty `a:p` element is added
        to ensure the cell is compliant with the spec.

def append_ps_from(self, spanned_tc):
        """Append `a:p` elements taken from *spanned_tc*.

        Any non-empty paragraph elements in *spanned_tc* are removed and
        appended to the text-frame of this cell. If *spanned_tc* is left with
        no content after this process, a single empty `a:p` element is added
        to ensure the cell is compliant with the spec.
        """
        source_txBody = spanned_tc.get_or_add_txBody()
        target_txBody = self.get_or_add_txBody()

        # ---if source is empty, there's nothing to do---
        if source_txBody.is_empty:
            return

        # ---a single empty paragraph in target is overwritten---
        if target_txBody.is_empty:
            target_txBody.clear_content()

        for p in source_txBody.p_lst:
            target_txBody.append(p)

        # ---neither source nor target can be left without ps---
        source_txBody.unclear_content()
        target_txBody.unclear_content()