@Override
    public boolean add(final E arg) {
        final int i = indexOf(arg);
        if (i >= 0) {
            return false;
        }
        append(arg);
        return true;
    }
Expose the index in the internal array of a given object. -1 is returned
if the object is not present in the internal array.

@param e
           an object to check whether exists in this set
@return the index of the argument e in the internal array, or -1 if the
        object is not present
vespajlib/src/main/java/com/yahoo/collections/TinyIdentitySet.java