@Override
    public Object reset(AggregatorState state) {
        state.set.clear();
        if (state.counter != null) {
            state.counter.clear();
        }
        Set returnSet = new HashSet();
        return returnSet;   // returning an empty set.
    }
The initialization method for FunctionExecutor

@param attributeExpressionExecutors are the executors of each attributes in the function
@param processingMode               query processing mode
@param outputExpectsExpiredEvents   is expired events sent as output
@param configReader                 this hold the {@link CountAttributeAggregatorExecutor} configuration reader.
@param siddhiQueryContext           Siddhi query runtime context
modules/siddhi-core/src/main/java/io/siddhi/core/query/selector/attribute/aggregator/UnionSetAttributeAggregatorExecutor.java