private void append(final Object arg) {
        if (size == entries.length) {
            grow();
        }
        entries[size++] = arg;
    }
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/ArraySet.java