def mask_span(text: str, span: str, mask='<CITE>') -> str:
    """Mask all occurences of `span` in `text` by replacing with `mask`

    :param text: Text to be processed
    :param span_to_mask: The specific span in `text` to mask
    :param mask: The mask to replace all occurrences of `span` with
    :return: 'masked'
    """
    return text.replace(span, mask)