@Override
        public IndexedBindings clone() {
            try {
                IndexedBindings clone = (IndexedBindings)super.clone();
                clone.setValues = new BitSet(nameToIndex.size());
                clone.doubleValues = new double[nameToIndex.size()];
                for (int i = 0; i < nameToIndex.size(); ++i) {
                    clone.doubleValues[i] = missingValue;
                }
                return clone;
            }
            catch (CloneNotSupportedException e) {
                throw new RuntimeException("Programming error");
            }
        }
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/AbstractArrayContext.java