/**
* Add a new num with an abstractNumID
* @return return NumId of the added num
*/
public BigInteger addNum(BigInteger abstractNumID){
CTNum ctNum = this.ctNumbering.addNewNum();
ctNum.addNewAbstractNumId();
ctNum.getAbstractNumId().setVal(abstractNumID);
ctNum.setNumId(BigInteger.valueOf(nums.size()+1));
XWPFNum num = new XWPFNum(ctNum, this);
nums.add(num);
return ctNum.getNumId();
}