public ArrayContext clone() {
        ArrayContext clone = (ArrayContext)super.clone();
        clone.values = new Value[nameToIndex().size()];
        Arrays.fill(clone.values, missingValue);
        return clone;
    }
Creates a clone of this context suitable for evaluating against the same ranking expression
in a different thread (i.e, name name to index map, different value set.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/evaluation/ArrayContext.java