presto-main/src/main/java/io/prestosql/sql/planner/plan/WindowNode.java/WindowNode/Specification
class WindowNode:        @JsonCreator
        public Specification(
                @JsonProperty("partitionBy") List<Symbol> partitionBy,
                @JsonProperty("orderingScheme") Optional<OrderingScheme> orderingScheme)
        {
            requireNonNull(partitionBy, "partitionBy is null");
            requireNonNull(orderingScheme, "orderingScheme is null");

            this.partitionBy = ImmutableList.copyOf(partitionBy);
            this.orderingScheme = requireNonNull(orderingScheme, "orderingScheme is null");
        }