/// <summary>
            /// Implements the visitor for the <see cref="XshtdSyntaxDefinition"/> object.
            /// </summary>
            /// <param name="syntax"></param>
            /// <returns></returns>
            public object VisitSyntaxDefinition(XshtdSyntaxDefinition syntax)
            {
                SyntaxDefinition c;

                if (syntax.Name != null)
                {
                    c = def.syntaxDefDict[syntax.Name];
                }
                else
                {
                    if (syntax.Extensions == null)
                    {
                        return(null);
                    }
                    else
                    {
                        c = new SyntaxDefinition(syntax.Name);
                    }
                }

                // Copy extensions to highlighting theme object
                foreach (var item in syntax.Extensions)
                {
                    c.Extensions.Add(item);
                }

                syntax.AcceptElements(this);

                return(c);
            }
            /// <summary>
            /// Implements the visitor for a named color (<see cref="XshtdColor"/> object)
            /// that is contained in a <see cref="XshtdSyntaxDefinition"/> object.
            ///
            /// Method checks if given color name is unique and adds the color into the internal
            /// collection of inique colors if it is.
            /// </summary>
            /// <param name="syntax"></param>
            /// <param name="color"></param>
            /// <returns>Always returns null. Throws a <see cref="HighlightingDefinitionInvalidException"/>
            /// if color name is a duplicate.</returns>
            public object VisitColor(XshtdSyntaxDefinition syntax, XshtdColor color)
            {
                if (color.Name != null)
                {
                    if (color.Name.Length == 0)
                    {
                        throw Error(color, "Name must not be the empty string");
                    }

                    if (syntax == null)
                    {
                        throw Error(syntax, "Syntax Definition for theme must not be null");
                    }

                    SyntaxDefinition synDef;
                    if (def.syntaxDefDict.TryGetValue(syntax.Name, out synDef) == false)
                    {
                        throw Error(syntax, "Themed Syntax Definition does not exist '" + syntax.Name + "'.");
                    }

                    if (synDef.ColorGet(color.Name) == null)
                    {
                        synDef.ColorAdd(new HighlightingColor()
                        {
                            Name = color.Name
                        });
                    }
                    else
                    {
                        throw Error(color, "Duplicate color name '" + color.Name + "'.");
                    }
                }

                return(null);
            }
            /// <summary>
            /// Implements the visitor for a named color (<see cref="XshtdColor"/> object)
            /// that is contained in a <see cref="XshtdSyntaxDefinition"/> object.
            /// </summary>
            /// <param name="syntax"></param>
            /// <param name="color"></param>
            /// <returns></returns>
            public object VisitColor(XshtdSyntaxDefinition syntax, XshtdColor color)
            {
                if (color.Name == null)
                {
                    throw Error(color, "Name must not be null");
                }

                if (color.Name.Length == 0)
                {
                    throw Error(color, "Name must not be the empty string");
                }

                if (syntax == null)
                {
                    throw Error(syntax, "Syntax Definition for theme must not be null");
                }

                SyntaxDefinition  synDef;
                HighlightingColor highColor;

                if (def.syntaxDefDict.TryGetValue(syntax.Name, out synDef) == false)
                {
                    throw Error(syntax, "Themed Syntax Definition does not exist '" + syntax.Name + "'.");
                }

                highColor = synDef.ColorGet(color.Name);
                if (highColor == null)
                {
                    highColor = new HighlightingColor()
                    {
                        Name = color.Name
                    };
                    synDef.ColorAdd(highColor);
                }

                highColor.Foreground = color.Foreground;
                highColor.Background = color.Background;
                highColor.Underline  = color.Underline;
                highColor.FontStyle  = color.FontStyle;
                highColor.FontWeight = color.FontWeight;

                return(highColor);
            }
            /// <summary>
            /// Implements the visitor for the <see cref="XshtdSyntaxDefinition"/> object.
            /// </summary>
            /// <param name="syntax"></param>
            /// <returns></returns>
            public object VisitSyntaxDefinition(XshtdSyntaxDefinition syntax)
            {
                if (syntax.Name != null)
                {
                    if (syntax.Name.Length == 0)
                    {
                        throw Error(syntax, "Name must not be the empty string");
                    }

                    if (def.syntaxDefDict.ContainsKey(syntax.Name))
                    {
                        throw Error(syntax, "Duplicate syntax definition name '" + syntax.Name + "'.");
                    }

                    def.syntaxDefDict.Add(syntax.Name, new SyntaxDefinition());
                }

                syntax.AcceptElements(this);

                return(null);
            }
 /// <summary>
 /// Creates a new XshdColor instance that is part of a <see cref="XshtdSyntaxDefinition"/>.
 /// </summary>
 public XshtdColor(XshtdSyntaxDefinition syntax)
 {
     _syntax = syntax;
 }