code: public BigInteger getBigInteger(String attrName) {
          BigDecimal bd = getNumber(attrName);
          return bd == null ? null : bd.toBigInteger();
      }
docs: Returns the value of the specified attribute in the current item as an
  <code>BigInteger</code>; or null if the attribute doesn't exist.
  
  @see #isNull(String) #isNull(String) to check if the attribute value is
  null.
  @see #isPresent(String) #isPresent(String) to check if the attribute
  value is present.
  
  @throws NumberFormatException
  if the attribute value is null or not a valid representation
  of a {@code BigDecimal}.