presto-parquet/src/main/java/io/prestosql/parquet/GroupField.java/GroupField/GroupField
class GroupField:    public GroupField(Type type, int repetitionLevel, int definitionLevel, boolean required, ImmutableList<Optional<Field>> children)
    {
        super(type, repetitionLevel, definitionLevel, required);
        this.children = requireNonNull(children, "children is required");
    }