/**
* Checks if the specified, non-empty item is a double. Returns the double or throws an exception.
*
* @param it item to be checked
* @return number
* @throws QueryException query exception
*/
private ANum toNumber(final Item it) throws QueryException {
if (it.type.isUntyped()) return Dbl.get(it.dbl(info));
if (it instanceof ANum) return (ANum) it;
throw numberError(this, it);
}
@Override
public Item item(final QueryContext qc, final InputInfo ii) throws QueryException {
final Item it = exprs[0].atomItem(qc, info);
return it == null ? null : Dbl.get(sqrt(toDouble(it)));
}