Set value of marX attribute on ``<a:tcPr>`` child element. If *marX*
        is |None|, the marX attribute is removed. *marX* is a string, one of
        ``('marL', 'marR', 'marT', 'marB')``.

def _set_marX(self, marX, value):
        """
        Set value of marX attribute on ``<a:tcPr>`` child element. If *marX*
        is |None|, the marX attribute is removed. *marX* is a string, one of
        ``('marL', 'marR', 'marT', 'marB')``.
        """
        if value is None and self.tcPr is None:
            return
        tcPr = self.get_or_add_tcPr()
        setattr(tcPr, marX, value)