public DoubleOnlyArrayContext clone() {
        return (DoubleOnlyArrayContext)super.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/DoubleOnlyArrayContext.java