Return a new ``<w:num>`` element having numId of *num_id* and having
        a ``<w:abstractNumId>`` child with val attribute set to
        *abstractNum_id*.

def new(cls, num_id, abstractNum_id):
        """
        Return a new ``<w:num>`` element having numId of *num_id* and having
        a ``<w:abstractNumId>`` child with val attribute set to
        *abstractNum_id*.
        """
        num = OxmlElement('w:num')
        num.numId = num_id
        abstractNumId = CT_DecimalNumber.new(
            'w:abstractNumId', abstractNum_id
        )
        num.append(abstractNumId)
        return num