public List<Expression> expressions() {
        return flatMapToList(args(), a -> {
            if (a == null) {
                return Collections.emptyList();
            }
            if (a instanceof Expression) {
                return Collections.singletonList((Expression) a);
            }
            if (a instanceof Collection) {
                return collectionToExprs((Collection) a);
            }
            return Collections.emptyList();
        }, /*ignoreNull*/ true);
    }
Returns all of the expressions present in this query plan operator.
indexr-query-opt/src/main/java/io/indexr/query/plan/QueryPlan.java