@Override
    public void clear() {
        size = 0;
    }
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