/// <summary>
        /// Fetches the message summaries for the messages between the two indexes (inclusive) that have a higher mod-sequence value than the one specified.
        /// </summary>
        /// <remarks>
        /// Fetches the message summaries for the messages between the two indexes (inclusive) that have a higher mod-sequence value than the one specified.
        /// </remarks>
        /// <returns>An enumeration of summaries for the requested messages.</returns>
        /// <param name="min">The minimum index.</param>
        /// <param name="max">The maximum index, or <c>-1</c> to specify no upper bound.</param>
        /// <param name="modseq">The mod-sequence value.</param>
        /// <param name="items">The message summary items to fetch.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <exception cref="System.ArgumentOutOfRangeException">
        /// <para><paramref name="min"/> is out of range.</para>
        /// <para>-or-</para>
        /// <para><paramref name="max"/> is out of range.</para>
        /// <para>-or-</para>
        /// <para><paramref name="items"/> is empty.</para>
        /// </exception>
        /// <exception cref="System.ObjectDisposedException">
        /// The <see cref="ImapClient"/> has been disposed.
        /// </exception>
        /// <exception cref="System.InvalidOperationException">
        /// <para>The <see cref="ImapClient"/> is not connected.</para>
        /// <para>-or-</para>
        /// <para>The <see cref="ImapClient"/> is not authenticated.</para>
        /// <para>-or-</para>
        /// <para>The folder is not currently open.</para>
        /// </exception>
        /// <exception cref="System.NotSupportedException">
        /// The <see cref="ImapFolder"/> does not support mod-sequences.
        /// </exception>
        /// <exception cref="System.OperationCanceledException">
        /// The operation was canceled via the cancellation token.
        /// </exception>
        /// <exception cref="System.IO.IOException">
        /// An I/O error occurred.
        /// </exception>
        /// <exception cref="ImapProtocolException">
        /// The server's response contained unexpected tokens.
        /// </exception>
        /// <exception cref="ImapCommandException">
        /// The server replied with a NO or BAD response.
        /// </exception>
        public override IList<IMessageSummary> Fetch(int min, int max, ulong modseq, MessageSummaryItems items, CancellationToken cancellationToken = default (CancellationToken))
        {
            if (min < 0 || min >= Count)
                throw new ArgumentOutOfRangeException ("min");

            if (max != -1 && max < min)
                throw new ArgumentOutOfRangeException ("max");

            if (items == MessageSummaryItems.None)
                throw new ArgumentOutOfRangeException ("items");

            if (!SupportsModSeq)
                throw new NotSupportedException ("The ImapFolder does not support mod-sequences.");

            CheckState (true, false);

            var query = FormatSummaryItems (items, null);
            var command = string.Format ("FETCH {0} {1} (CHANGEDSINCE {2})\r\n", GetFetchRange (min, max), query, modseq);
            var ic = new ImapCommand (Engine, cancellationToken, this, command);
            var results = new SortedDictionary<int, IMessageSummary> ();
            ic.RegisterUntaggedHandler ("FETCH", FetchSummaryItems);
            ic.UserData = results;

            Engine.QueueCommand (ic);
            Engine.Wait (ic);

            ProcessResponseCodes (ic, null);

            if (ic.Result != ImapCommandResult.Ok)
                throw ImapCommandException.Create ("FETCH", ic);

            return AsReadOnly (results.Values);
        }

		/// <summary>
		/// Fetches the message summaries for the specified message indexes that have a
		/// higher mod-sequence value than the one specified.
		/// </summary>
		/// <remarks>
		/// <para>Fetches the message summaries for the specified message indexes that
		/// have a higher mod-sequence value than the one specified.</para>
		/// <para>It should be noted that if another client has modified any message
		/// in the folder, the IMAP server may choose to return information that was
		/// not explicitly requested. It is therefore important to be prepared to
		/// handle both additional fields on a <see cref="IMessageSummary"/> for
		/// messages that were requested as well as summaries for messages that were
		/// not requested at all.</para>
		/// </remarks>
		/// <returns>An enumeration of summaries for the requested messages.</returns>
		/// <param name="indexes">The indexes.</param>
		/// <param name="modseq">The mod-sequence value.</param>
		/// <param name="items">The message summary items to fetch.</param>
		/// <param name="cancellationToken">The cancellation token.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="indexes"/> is <c>null</c>.
		/// </exception>
		/// <exception cref="System.ArgumentOutOfRangeException">
		/// <paramref name="items"/> is empty.
		/// </exception>
		/// <exception cref="System.ArgumentException">
		/// One or more of the <paramref name="indexes"/> is invalid.
		/// </exception>
		/// <exception cref="System.ObjectDisposedException">
		/// The <see cref="ImapClient"/> has been disposed.
		/// </exception>
		/// <exception cref="ServiceNotConnectedException">
		/// The <see cref="ImapClient"/> is not connected.
		/// </exception>
		/// <exception cref="ServiceNotAuthenticatedException">
		/// The <see cref="ImapClient"/> is not authenticated.
		/// </exception>
		/// <exception cref="FolderNotOpenException">
		/// The <see cref="ImapFolder"/> is not currently open.
		/// </exception>
		/// <exception cref="System.NotSupportedException">
		/// The <see cref="ImapFolder"/> does not support mod-sequences.
		/// </exception>
		/// <exception cref="System.OperationCanceledException">
		/// The operation was canceled via the cancellation token.
		/// </exception>
		/// <exception cref="System.IO.IOException">
		/// An I/O error occurred.
		/// </exception>
		/// <exception cref="ImapProtocolException">
		/// The server's response contained unexpected tokens.
		/// </exception>
		/// <exception cref="ImapCommandException">
		/// The server replied with a NO or BAD response.
		/// </exception>
		public override IList<IMessageSummary> Fetch (IList<int> indexes, ulong modseq, MessageSummaryItems items, CancellationToken cancellationToken = default (CancellationToken))
		{
			var set = ImapUtils.FormatIndexSet (indexes);

			if (items == MessageSummaryItems.None)
				throw new ArgumentOutOfRangeException ("items");

			if (!SupportsModSeq)
				throw new NotSupportedException ("The ImapFolder does not support mod-sequences.");

			CheckState (true, false);

			if (indexes.Count == 0)
				return new IMessageSummary[0];

			var query = FormatSummaryItems (ref items, null);
			var command = string.Format ("FETCH {0} {1} (CHANGEDSINCE {2})\r\n", set, query, modseq);
			var ic = new ImapCommand (Engine, cancellationToken, this, command);
			var ctx = new FetchSummaryContext (items);

			ic.RegisterUntaggedHandler ("FETCH", FetchSummaryItems);
			ic.UserData = ctx;

			Engine.QueueCommand (ic);
			Engine.Wait (ic);

			ProcessResponseCodes (ic, null);

			if (ic.Response != ImapCommandResponse.Ok)
				throw ImapCommandException.Create ("FETCH", ic);

			return AsReadOnly (ctx.Results.Values);
		}

		/// <summary>
		/// Fetch the message summaries for the messages between the two indexes (inclusive)
		/// that have a higher mod-sequence value than the one specified.
		/// </summary>
		/// <remarks>
		/// <para>Fetches the message summaries for the messages between the two
		/// indexes (inclusive) that have a higher mod-sequence value than the one
		/// specified.</para>
		/// <para>It should be noted that if another client has modified any message
		/// in the folder, the mail service may choose to return information that was
		/// not explicitly requested. It is therefore important to be prepared to
		/// handle both additional fields on a <see cref="IMessageSummary"/> for
		/// messages that were requested as well as summaries for messages that were
		/// not requested at all.</para>
		/// </remarks>
		/// <returns>An enumeration of summaries for the requested messages.</returns>
		/// <param name="min">The minimum index.</param>
		/// <param name="max">The maximum index, or <c>-1</c> to specify no upper bound.</param>
		/// <param name="modseq">The mod-sequence value.</param>
		/// <param name="items">The message summary items to fetch.</param>
		/// <param name="fields">The desired header fields.</param>
		/// <param name="cancellationToken">The cancellation token.</param>
		/// <exception cref="System.ArgumentOutOfRangeException">
		/// <para><paramref name="min"/> is out of range.</para>
		/// <para>-or-</para>
		/// <para><paramref name="max"/> is out of range.</para>
		/// </exception>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="fields"/> is <c>null</c>.
		/// </exception>
		/// <exception cref="System.ArgumentException">
		/// <paramref name="fields"/> is empty.
		/// </exception>
		/// <exception cref="System.ObjectDisposedException">
		/// The <see cref="IMailStore"/> has been disposed.
		/// </exception>
		/// <exception cref="ServiceNotConnectedException">
		/// The <see cref="IMailStore"/> is not connected.
		/// </exception>
		/// <exception cref="ServiceNotAuthenticatedException">
		/// The <see cref="IMailStore"/> is not authenticated.
		/// </exception>
		/// <exception cref="FolderNotOpenException">
		/// The folder is not currently open.
		/// </exception>
		/// <exception cref="System.NotSupportedException">
		/// The <see cref="MailFolder"/> does not support mod-sequences.
		/// </exception>
		/// <exception cref="System.OperationCanceledException">
		/// The operation was canceled via the cancellation token.
		/// </exception>
		/// <exception cref="System.IO.IOException">
		/// An I/O error occurred.
		/// </exception>
		/// <exception cref="ProtocolException">
		/// The server's response contained unexpected tokens.
		/// </exception>
		/// <exception cref="CommandException">
		/// The command failed.
		/// </exception>
		public abstract IList<IMessageSummary> Fetch (int min, int max, ulong modseq, MessageSummaryItems items, HashSet<string> fields, CancellationToken cancellationToken = default (CancellationToken));

			public FetchSummaryContext (MessageSummaryItems requestedItems)
			{
				Results = new SortedDictionary<int, IMessageSummary> ();
				RequestedItems = requestedItems;
			}

        /// <summary>
        /// Fetches the message summaries for the specified message indexes that have a higher mod-sequence value than the one specified.
        /// </summary>
        /// <returns>An enumeration of summaries for the requested messages.</returns>
        /// <param name="indexes">The indexes.</param>
        /// <param name="modseq">The mod-sequence value.</param>
        /// <param name="items">The message summary items to fetch.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <exception cref="System.ArgumentNullException">
        /// <paramref name="indexes"/> is <c>null</c>.
        /// </exception>
        /// <exception cref="System.ArgumentOutOfRangeException">
        /// <paramref name="items"/> is empty.
        /// </exception>
        /// <exception cref="System.ArgumentException">
        /// One or more of the <paramref name="indexes"/> is invalid.
        /// </exception>
        /// <exception cref="System.ObjectDisposedException">
        /// The <see cref="ImapClient"/> has been disposed.
        /// </exception>
        /// <exception cref="System.InvalidOperationException">
        /// <para>The <see cref="ImapClient"/> is not connected.</para>
        /// <para>-or-</para>
        /// <para>The <see cref="ImapClient"/> is not authenticated.</para>
        /// <para>-or-</para>
        /// <para>The folder is not currently open.</para>
        /// </exception>
        /// <exception cref="System.NotSupportedException">
        /// The <see cref="ImapFolder"/> does not support mod-sequences.
        /// </exception>
        /// <exception cref="System.OperationCanceledException">
        /// The operation was canceled via the cancellation token.
        /// </exception>
        /// <exception cref="System.IO.IOException">
        /// An I/O error occurred.
        /// </exception>
        /// <exception cref="ImapProtocolException">
        /// The server's response contained unexpected tokens.
        /// </exception>
        /// <exception cref="ImapCommandException">
        /// The server replied with a NO or BAD response.
        /// </exception>
        public IEnumerable<MessageSummary> Fetch(int[] indexes, ulong modseq, MessageSummaryItems items, CancellationToken cancellationToken)
        {
            var set = ImapUtils.FormatIndexSet (indexes);
            var query = FormatSummaryItems (items);

            if (items == MessageSummaryItems.None)
                throw new ArgumentOutOfRangeException ("items");

            if (!SupportsModSeq)
                throw new NotSupportedException ("The ImapFolder does not support mod-sequences.");

            CheckState (true, false);

            if (indexes.Length == 0)
                return new MessageSummary[0];

            var command = string.Format ("FETCH {0} ({1}) (CHANGEDSINCE {2})\r\n", set, query, modseq);
            var ic = Engine.QueueCommand (cancellationToken, this, command);
            var results = new SortedDictionary<int, MessageSummary> ();
            ic.RegisterUntaggedHandler ("FETCH", FetchSummaryItems);
            ic.UserData = results;

            Engine.Wait (ic);

            ProcessResponseCodes (ic, null);

            if (ic.Result != ImapCommandResult.Ok)
                throw new ImapCommandException ("FETCH", ic.Result);

            return results.Values;
        }

        /// <summary>
        /// Fetches the message summaries for the messages between the two UIDs (inclusive) that have a higher mod-sequence value than the one specified.
        /// </summary>
        /// <remarks>
        /// <para>If the IMAP server supports the QRESYNC extension and the application has
        /// enabled this feature via <see cref="ImapClient.EnableQuickResync"/>, then this
        /// method will emit <see cref="Vanished"/> events for messages that have vanished
        /// since the specified mod-sequence value.</para>
        /// </remarks>
        /// <returns>An enumeration of summaries for the requested messages.</returns>
        /// <param name="min">The minimum UID.</param>
        /// <param name="max">The maximum UID.</param>
        /// <param name="modseq">The mod-sequence value.</param>
        /// <param name="items">The message summary items to fetch.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <exception cref="System.ArgumentException">
        /// <paramref name="min"/> is invalid.
        /// </exception>
        /// <exception cref="System.ArgumentOutOfRangeException">
        /// <paramref name="items"/> is empty.
        /// </exception>
        /// <exception cref="System.ObjectDisposedException">
        /// The <see cref="ImapClient"/> has been disposed.
        /// </exception>
        /// <exception cref="System.InvalidOperationException">
        /// <para>The <see cref="ImapClient"/> is not connected.</para>
        /// <para>-or-</para>
        /// <para>The <see cref="ImapClient"/> is not authenticated.</para>
        /// <para>-or-</para>
        /// <para>The folder is not currently open.</para>
        /// </exception>
        /// <exception cref="System.NotSupportedException">
        /// The <see cref="ImapFolder"/> does not support mod-sequences.
        /// </exception>
        /// <exception cref="System.OperationCanceledException">
        /// The operation was canceled via the cancellation token.
        /// </exception>
        /// <exception cref="System.IO.IOException">
        /// An I/O error occurred.
        /// </exception>
        /// <exception cref="ImapProtocolException">
        /// The server's response contained unexpected tokens.
        /// </exception>
        /// <exception cref="ImapCommandException">
        /// The server replied with a NO or BAD response.
        /// </exception>
        public IEnumerable<MessageSummary> Fetch(UniqueId min, UniqueId? max, ulong modseq, MessageSummaryItems items, CancellationToken cancellationToken)
        {
            if (min.Id == 0)
                throw new ArgumentException ("The minimum uid is invalid.", "min");

            if (items == MessageSummaryItems.None)
                throw new ArgumentOutOfRangeException ("items");

            if (!SupportsModSeq)
                throw new NotSupportedException ("The ImapFolder does not support mod-sequences.");

            CheckState (true, false);

            var query = FormatSummaryItems (items);
            var maxValue = max.HasValue ? max.Value.Id.ToString () : "*";
            var vanished = Engine.QResyncEnabled ? " VANISHED" : string.Empty;
            var command = string.Format ("UID FETCH {0}:{1} ({2}) (CHANGEDSINCE {3}{4})\r\n", min.Id, maxValue, query, modseq, vanished);
            var ic = new ImapCommand (Engine, cancellationToken, this, command);
            var results = new SortedDictionary<int, MessageSummary> ();
            ic.RegisterUntaggedHandler ("FETCH", FetchSummaryItems);
            ic.UserData = results;

            Engine.QueueCommand (ic);
            Engine.Wait (ic);

            ProcessResponseCodes (ic, null);

            if (ic.Result != ImapCommandResult.Ok)
                throw new ImapCommandException ("FETCH", ic.Result);

            return results.Values;
        }

		/// <summary>
		/// Asynchronously fetch the message summaries for the specified message UIDs.
		/// </summary>
		/// <remarks>
		/// <para>Asynchronously fetches the message summaries for the specified message
		/// UIDs.</para>
		/// <para>It should be noted that if another client has modified any message
		/// in the folder, the mail service may choose to return information that was
		/// not explicitly requested. It is therefore important to be prepared to
		/// handle both additional fields on a <see cref="IMessageSummary"/> for
		/// messages that were requested as well as summaries for messages that were
		/// not requested at all.</para>
		/// </remarks>
		/// <returns>An enumeration of summaries for the requested messages.</returns>
		/// <param name="uids">The UIDs.</param>
		/// <param name="items">The message summary items to fetch.</param>
		/// <param name="fields">The desired header fields.</param>
		/// <param name="cancellationToken">The cancellation token.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <para><paramref name="uids"/> is <c>null</c>.</para>
		/// <para>-or-</para>
		/// <para><paramref name="fields"/> is <c>null</c>.</para>
		/// </exception>
		/// <exception cref="System.ArgumentException">
		/// <para>One or more of the <paramref name="uids"/> is invalid.</para>
		/// <para>-or-</para>
		/// <para><paramref name="fields"/> is empty.</para>
		/// </exception>
		/// <exception cref="System.ObjectDisposedException">
		/// The <see cref="IMailStore"/> has been disposed.
		/// </exception>
		/// <exception cref="ServiceNotConnectedException">
		/// The <see cref="IMailStore"/> is not connected.
		/// </exception>
		/// <exception cref="ServiceNotAuthenticatedException">
		/// The <see cref="IMailStore"/> is not authenticated.
		/// </exception>
		/// <exception cref="FolderNotOpenException">
		/// The folder is not currently open.
		/// </exception>
		/// <exception cref="System.OperationCanceledException">
		/// The operation was canceled via the cancellation token.
		/// </exception>
		/// <exception cref="System.IO.IOException">
		/// An I/O error occurred.
		/// </exception>
		/// <exception cref="ProtocolException">
		/// The server's response contained unexpected tokens.
		/// </exception>
		/// <exception cref="CommandException">
		/// The command failed.
		/// </exception>
		public virtual Task<IList<IMessageSummary>> FetchAsync (IList<UniqueId> uids, MessageSummaryItems items, HashSet<string> fields, CancellationToken cancellationToken = default (CancellationToken))
		{
			if (uids == null)
				throw new ArgumentNullException (nameof (uids));

			if (fields == null)
				throw new ArgumentNullException (nameof (fields));

			if (fields.Count == 0)
				throw new ArgumentException ("The set of header fields cannot be empty.", nameof (fields));

			return Task.Factory.StartNew (() => {
				lock (SyncRoot) {
					return Fetch (uids, items, fields, cancellationToken);
				}
			}, cancellationToken, TaskCreationOptions.None, TaskScheduler.Default);
		}

        string FormatSummaryItems(MessageSummaryItems items)
        {
            string query;

            if ((items & MessageSummaryItems.BodyStructure) != 0 && (items & MessageSummaryItems.Body) != 0) {
                // don't query both the BODY and BODYSTRUCTURE, that's just dumb...
                items &= ~MessageSummaryItems.Body;
            }

            // Note: GMail doesn't properly handle aliases (or at least it doesn't handle "FULL")...
            if ((Engine.Capabilities & ImapCapabilities.GMailExt1) == 0) {
                // first, eliminate the aliases...
                if ((items & MessageSummaryItems.Full) == MessageSummaryItems.Full) {
                    items &= ~MessageSummaryItems.Full;
                    query = "FULL ";
                } else if ((items & MessageSummaryItems.All) == MessageSummaryItems.All) {
                    items &= ~MessageSummaryItems.All;
                    query = "ALL ";
                } else if ((items & MessageSummaryItems.Fast) == MessageSummaryItems.Fast) {
                    items &= ~MessageSummaryItems.Fast;
                    query = "FAST ";
                } else {
                    query = string.Empty;
                }
            } else {
                query = string.Empty;
            }

            // now add on any additional summary items...
            if ((items & MessageSummaryItems.Uid) != 0)
                query += "UID ";
            if ((items & MessageSummaryItems.Flags) != 0)
                query += "FLAGS ";
            if ((items & MessageSummaryItems.InternalDate) != 0)
                query += "INTERNALDATE ";
            if ((items & MessageSummaryItems.MessageSize) != 0)
                query += "RFC822.SIZE ";
            if ((items & MessageSummaryItems.Envelope) != 0)
                query += "ENVELOPE ";
            if ((items & MessageSummaryItems.BodyStructure) != 0)
                query += "BODYSTRUCTURE ";
            if ((items & MessageSummaryItems.Body) != 0)
                query += "BODY ";

            if ((Engine.Capabilities & ImapCapabilities.CondStore) != 0) {
                if ((items & MessageSummaryItems.ModSeq) != 0)
                    query += "MODSEQ ";
            }

            if ((Engine.Capabilities & ImapCapabilities.GMailExt1) != 0) {
                // now for the GMail extension items
                if ((items & MessageSummaryItems.GMailMessageId) != 0)
                    query += "X-GM-MSGID ";
                if ((items & MessageSummaryItems.GMailThreadId) != 0)
                    query += "X-GM-THRID ";
            }

            return query.TrimEnd ();
        }

		/// <summary>
		/// Asynchronously fetch the message summaries for the messages between the two indexes
		/// (inclusive) that have a higher mod-sequence value than the one specified.
		/// </summary>
		/// <remarks>
		/// <para>Asynchronously fetches the message summaries for the messages between
		/// the two indexes (inclusive) that have a higher mod-sequence value than the
		/// one specified.</para>
		/// <para>It should be noted that if another client has modified any message
		/// in the folder, the mail service may choose to return information that was
		/// not explicitly requested. It is therefore important to be prepared to
		/// handle both additional fields on a <see cref="IMessageSummary"/> for
		/// messages that were requested as well as summaries for messages that were
		/// not requested at all.</para>
		/// </remarks>
		/// <returns>An enumeration of summaries for the requested messages.</returns>
		/// <param name="min">The minimum index.</param>
		/// <param name="max">The maximum index, or <c>-1</c> to specify no upper bound.</param>
		/// <param name="modseq">The mod-sequence value.</param>
		/// <param name="items">The message summary items to fetch.</param>
		/// <param name="cancellationToken">The cancellation token.</param>
		/// <exception cref="System.ArgumentOutOfRangeException">
		/// <para><paramref name="min"/> is out of range.</para>
		/// <para>-or-</para>
		/// <para><paramref name="max"/> is out of range.</para>
		/// <para>-or-</para>
		/// <para><paramref name="items"/> is empty.</para>
		/// </exception>
		/// <exception cref="System.ObjectDisposedException">
		/// The <see cref="IMailStore"/> has been disposed.
		/// </exception>
		/// <exception cref="ServiceNotConnectedException">
		/// The <see cref="IMailStore"/> is not connected.
		/// </exception>
		/// <exception cref="ServiceNotAuthenticatedException">
		/// The <see cref="IMailStore"/> is not authenticated.
		/// </exception>
		/// <exception cref="FolderNotOpenException">
		/// The folder is not currently open.
		/// </exception>
		/// <exception cref="System.NotSupportedException">
		/// The <see cref="MailFolder"/> does not support mod-sequences.
		/// </exception>
		/// <exception cref="System.OperationCanceledException">
		/// The operation was canceled via the cancellation token.
		/// </exception>
		/// <exception cref="System.IO.IOException">
		/// An I/O error occurred.
		/// </exception>
		/// <exception cref="ProtocolException">
		/// The server's response contained unexpected tokens.
		/// </exception>
		/// <exception cref="CommandException">
		/// The command failed.
		/// </exception>
		public virtual Task<IList<IMessageSummary>> FetchAsync (int min, int max, ulong modseq, MessageSummaryItems items, CancellationToken cancellationToken = default (CancellationToken))
		{
			if (min < 0)
				throw new ArgumentOutOfRangeException ("min");

			if (max != -1 && max < min)
				throw new ArgumentOutOfRangeException ("max");

			if (items == MessageSummaryItems.None)
				throw new ArgumentOutOfRangeException ("items");

			return Task.Factory.StartNew (() => {
				lock (SyncRoot) {
					return Fetch (min, max, modseq, items, cancellationToken);
				}
			}, cancellationToken, TaskCreationOptions.None, TaskScheduler.Default);
		}

		string FormatSummaryItems (ref MessageSummaryItems items, HashSet<string> fields)
		{
			if ((items & MessageSummaryItems.BodyStructure) != 0 && (items & MessageSummaryItems.Body) != 0) {
				// don't query both the BODY and BODYSTRUCTURE, that's just dumb...
				items &= ~MessageSummaryItems.Body;
			}

			if (!Engine.IsGMail) {
				// first, eliminate the aliases...
				if (items == MessageSummaryItems.All)
					return "ALL";

				if (items == MessageSummaryItems.Full)
					return "FULL";

				if (items == MessageSummaryItems.Fast)
					return "FAST";
			}

			var tokens = new List<string> ();

			// now add on any additional summary items...
			if ((items & MessageSummaryItems.UniqueId) != 0)
				tokens.Add ("UID");
			if ((items & MessageSummaryItems.Flags) != 0)
				tokens.Add ("FLAGS");
			if ((items & MessageSummaryItems.InternalDate) != 0)
				tokens.Add ("INTERNALDATE");
			if ((items & MessageSummaryItems.MessageSize) != 0)
				tokens.Add ("RFC822.SIZE");
			if ((items & MessageSummaryItems.Envelope) != 0)
				tokens.Add ("ENVELOPE");
			if ((items & MessageSummaryItems.BodyStructure) != 0)
				tokens.Add ("BODYSTRUCTURE");
			if ((items & MessageSummaryItems.Body) != 0)
				tokens.Add ("BODY");

			if ((Engine.Capabilities & ImapCapabilities.CondStore) != 0) {
				if ((items & MessageSummaryItems.ModSeq) != 0)
					tokens.Add ("MODSEQ");
			}

			if ((Engine.Capabilities & ImapCapabilities.GMailExt1) != 0) {
				// now for the GMail extension items
				if ((items & MessageSummaryItems.GMailMessageId) != 0)
					tokens.Add ("X-GM-MSGID");
				if ((items & MessageSummaryItems.GMailThreadId) != 0)
					tokens.Add ("X-GM-THRID");
				if ((items & MessageSummaryItems.GMailLabels) != 0)
					tokens.Add ("X-GM-LABELS");
			}

			if ((items & MessageSummaryItems.References) != 0 || fields != null) {
				var headers = new StringBuilder ("BODY.PEEK[HEADER.FIELDS (");
				bool references = false;

				if (fields != null) {
					foreach (var field in fields) {
						var name = field.ToUpperInvariant ();

						if (name == "REFERENCES")
							references = true;

						headers.Append (name);
						headers.Append (' ');
					}
				}

				if ((items & MessageSummaryItems.References) != 0 && !references)
					headers.Append ("REFERENCES ");

				headers[headers.Length - 1] = ')';
				headers.Append (']');

				tokens.Add (headers.ToString ());
			}

			if (tokens.Count == 1)
				return tokens[0];

			return string.Format ("({0})", string.Join (" ", tokens));
		}