Peach Pits of GIF Model

I've been studying about peach 3 these days. Below is my first Peach Pit about GIF. There're several dissimilarities of version 3 vs 2. When writing this pit, I found one of the most obvious change is that there is no 'when' attribute in version 3, we should use 'choice' or 'constraint' instead.  Here is my pit, sorry to annotated little, ask me if you have any question or doubts:)

<?xml version="1.0" encoding="UTF-8"?>
<!--  
    Author:@Puzzor Weibo;@Puzzorsj twitter
    Date:2013.09.07
    Type:GIF
-->
<Peach xmlns="http://peachfuzzer.com/2012/Peach"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://peachfuzzer.com/2012/Peach file:/C:/Peach/peach.xsd">
    <Import import="math"/>
    <!-- Several Templates to ref-->
    <DataModel name="LogicalScreenDescriptorTemplate">
        <Number size="16" name="Width"></Number>
        <Number size="16" name="Height"></Number>
        <Number size="1" name="GlobalColor"/>
        <Number size="3" name="ColorResolution"/>
        <Number size="1" name="SortFlag"/>
        <Number size="3" name="SizeofGlobalColorTable" />
        <Number size="8" name="BackgroundColorIndex"></Number>
        <Number size="8" name="PixelAspectRatio"></Number>
    </DataModel>  
    <DataModel name="ImageDescriptorTemplate">
        <Blob name="ImageSeparator" length="1" value="2c" valueType="hex" mutable="false" token="true"></Blob>
        <Number size="16" name="ImageLeftPosition"/>
        <Number size="16" name="ImageTopPosition"/>
        <Number size="16" name="ImageWidth"/>
        <Number size="16" name="ImageHeight"/>
        <Number size="1" name="LocalColorTableFlag"/>
        <Number size="1" name="InterlaceFlag"/>
        <Number size="1" name="SortFlag"/>
        <Number size="2" name="Reserved"/>
        <Number size="3" name="SizeOfLocalColorTable"/>
    </DataModel>
    <DataModel name="DataSubBlocksTemplate">
        <Choice name="ChoiceOfDataSubBlocksSize" minOccurs="0">
            <Block>
                <Block name="DataSubBlocksWithSizeNotZero" >
                    <Number size="8" name="BlockSize" signed="false" constraint="int(element.DefaultValue) != 0">
                        <Relation type="size" of="DataValues"/>
                    </Number>
                    <Blob name="DataValues"/>
                </Block>
            </Block>
        </Choice>
        <Blob name="BlockTerminator" length="1" valueType="hex" value="00" mutable="false" token="true"/>
    </DataModel>
    <DataModel name="ExtensionHeaderTemplate">
        <Blob length="1" name="ExtensionIntroducer" value="21" valueType="hex" mutable="false" token="true"/>
        <Blob length="1" name="ExtensionLabel" valueType="hex" mutable="false" token="true"/>
    </DataModel>
    <!-- ################   Template   ################# -->
    <DataModel name="GIFHeader" mutable="false" minOccurs="1" maxOccurs="1">
        <String length="3" value="GIF" mutable="false" token="true"/>
        <Blob name="strVersion" length="3"/>
    </DataModel>
    <DataModel name="LogicalScreenDescriptorModel">
        <Choice name="GlobalColorChoice">
            <Block name="ChoiceOfGlobalColorT">
                <Block name="LogicalScreenDescriptorREF" ref="LogicalScreenDescriptorTemplate">
                    <Number size="1" name="GlobalColor" value="1"/>
                    <Number size="3" name="SizeofGlobalColorTable">
                        <Relation type="size" of="GlobalColorTableBlob" expressionGet="int(pow(2,(size+1)))*3" expressionSet="int(math.log(size/3,2))-1"/>
                    </Number>
                    <Blob name="GlobalColorTableBlob" maxOccurs="1"/>
                </Block>
            </Block>
            <Block name="ChoiceOfGlobalColorF" ref="LogicalScreenDescriptorTemplate">
                <Number size="1" name="GlobalColor" value="0"></Number>
            </Block>
        </Choice>  
    </DataModel>
    <DataModel name="ImageData">
        <Number size="8" name="LZWMinimumCodeSize"/>
        <Block name="ImageData" ref="DataSubBlocksTemplate"></Block>
    </DataModel>
    <DataModel name="ImageDescriptor">
        <Choice name="ImageDescriptorLocalColorTableChoice">
            <Block name="IDLocalColorTableT" ref="ImageDescriptorTemplate">
                <Number size="1" name="LocalColorTableFlag" value="1" token="true"></Number>
                <Number size="3" name="SizeOfLocalColorTable">
                    <Relation type="size" of="LocalColorTableBlob" expressionGet="int(pow(2,(size+1)))*3" expressionSet="int(math.log(size/3,2))-1"/>
                </Number>
                <Blob name="LocalColorTableBlob"/>
            </Block>
            <Block name="IDLocalColorTableF" ref="ImageDescriptorTemplate">
                <Number size="1" name="LocalColorTableFlag" value="0" token="true"></Number>
            </Block>
        </Choice>
        <Block ref="ImageData"/>
    </DataModel>
    <DataModel name="GraphicControlExtension">
        <Block name="ExtensionHeader" ref="ExtensionHeaderTemplate">
            <Blob name="ExtensionLabel" length="1" valueType="hex" value="F9" mutable="false" token="true"></Blob>
        </Block>
        <Block name="GraphicControlSubBlock">
            <Number size="8" name="BlockSize"></Number>
            <Flags name="PackedFields" size="8">
                <Flag name="Reserved" position="5" size="3"/>
                <Flag name="DisposalMethod" position="2" size="3"/>
                <Flag name="UserInputFlag" position="1" size="1"/>
                <Flag name="TransparentColorFlag" position="0" size="1"/>
            </Flags>
            <Number size="16" name="DelayTime"/>
            <Number size="8" name="TransparentColorIndex"/>
        </Block>
        <Blob name="BlockTerminator" length="1" valueType="hex" value="00" mutable="false"></Blob>
    </DataModel>
    <DataModel name="PlainTextExtension">
        <Block name="ExtensionHeader" ref="ExtensionHeaderTemplate">
            <Blob name="ExtensionLabel" value="01" length="1" valueType="hex" token="true" mutable="false"></Blob>
        </Block>
        <Block name="PlainText">
            <Number size="8" name="BlockSize"></Number>
            <Number size="16" name="TextGridLeftPosition"></Number>
            <Number size="16" name="TextGridTopPosition"></Number>
            <Number size="16" name="TextGridWidth"></Number>
            <Number size="16" name="TextGridHeight"></Number>
            <Number size="8" name="CharacterCellWidth"></Number>
            <Number size="8" name="CharacterCellHeight"></Number>
            <Number size="8" name="TextForegroundColorIndex"></Number>
            <Number size="8" name="TextBackgroundColorIndex"></Number>
        </Block>
        <Block name="DataSubBlocks" ref="DataSubBlocksTemplate"></Block>
    </DataModel>
    <DataModel name="CommentExtension">
        <Block name="ExtensionHeader" ref="ExtensionHeaderTemplate">
            <Blob name="ExtensionLabel" valueType="hex" value="FE" length="1" token="true" mutable="false"/>
        </Block>
        <Block name="DataSubBlocks" ref="DataSubBlocksTemplate"/>
    </DataModel>
    <DataModel name="ApplicationExtension">
        <Block name="ExtensionHeader" ref="ExtensionHeaderTemplate">
            <Blob name="ExtensionLabel" valueType="hex" value="FF" length="1" token="true" mutable="false"/>
        </Block>
        <Block name="ApplicationEntensionBlock">
            <Number size="8" name="BlockSize"></Number>
            <Blob name="ApplicationIdentifier" length="8"></Blob>
            <Blob name="ApplicationAuthenticationCode" length="3"></Blob>
        </Block>
        <Block name="DataSubBlocks" ref="DataSubBlocksTemplate" />
    </DataModel>
    <DataModel name="UndefinedData">
        <Block name="ExtensionHeader" ref="ExtensionHeaderTemplate">
            <Blob name="ExtensionLabel" length="1" valueType="hex" value="F9" token="true" mutable="false"/>
        </Block>
        <Block name="DataSubBlocks" ref="DataSubBlocksTemplate"/>
    </DataModel>
    <DataModel name="GIFTrailer">
        <Blob name="GIFTrailer" value="3B" valueType="hex" mutable="false" length="1" token="true"></Blob>
    </DataModel>
    <DataModel name ="Data">
        <Choice minOccurs="1">
            <Block ref="ImageDescriptor"/>
            <Block ref="GraphicControlExtension"/>
            <Block ref="CommentExtension"/>
            <Block ref="PlainTextExtension"/>
            <Block ref="ApplicationExtension"/>
            <Block ref="UndefinedData"/>
        </Choice>
    </DataModel>
    <DataModel name="GIF" >
        <Block ref="GIFHeader"></Block>
        <Block ref="LogicalScreenDescriptorModel"></Block>
        <Block ref="Data"></Block>
        <Block ref="GIFTrailer"></Block>
    </DataModel>
    <StateModel name="TheState" initialState="Initial">
        <State name="Initial">
            <Action type="output">
                <DataModel ref="GIF"/>
                <Data fileName="sample.gif"/>
            </Action>
            <Action type="close"/>
        </State>
    </StateModel>
    <Test name="Default">
        <StateModel ref="TheState"/>
        <Publisher class="File">
            <Param name="FileName" value="fuzzed.gif"/>
        </Publisher>
        <Logger class="Filesystem">
            <Param name="Path" value="logs" />
        </Logger>
    </Test>
</Peach>