How to load code using ant script based on baseline ?

I've been able to fetch all code(to be clear latest) using ant. What I want is to fetch the code incrementally.
Here's the script I use for now. What should I do to change the script to fetch code that delivered to stream since last baseline?

test.bat is the script to kick off ant.
@echo off
set ANT_HOME=C:\test_0503\apache-ant-1.9.4
set JAVA_HOME=C:\Program Files (x86)\Java\jdk1.7.0_01
set PATH=%ANT_HOME%\bin;%JAVA_HOME%\bin;
ant -lib C:\test_0503\buildtoolkit -buildfile=C:\test_0503\build.xml -l C:\test_0503\build.log
echo "Start build..."
pause
The content of build.xml is as below, I used 2 tasks:teamFetch and teamAccept:

<project name="simpleRTCAntBuildExample" default="rtcFetchCode" basedir=".">

<import file="C:\test_0503\buildtoolkit\BuildToolkitTaskDefs.xml"/>

<taskdef name="teamFetch" classname="com.ibm.team.build.ant.task.TeamFetchTask" />
<taskdef name="teamAccept" classname="com.ibm.team.build.ant.task.TeamAcceptTask"/>   

<target name="init">
    <property name="repositoryAddress" value="https://www.hdyhuang.com:9443/ccm/" />
    <property name="rtc.username" value="admin1" />
    <property name="rtc.password" value="admin1" />
    <property name="rtc.workspace.name" value="project area_2.7.0_model Workspace" />
    <property name="deploy.checkout.dir" value="C:\test_0503\dir" />
</target>

<target name="rtcFetchCode"  depends="init">
    <teamAccept repositoryAddress="${repositoryAddress}"
                userId="${rtc.username}"
                password="${rtc.password}"
                workspaceName="${rtc.workspace.name}"
                changeSetFile="changeSets.txt"
                verbose="true" />
    <echo message="export from test_build_workspace"/>
    <teamFetch repositoryAddress="${repositoryAddress}"
                            userId="${rtc.username}"
                            password="${rtc.password}"
                            workspaceName="${rtc.workspace.name}"
                            destination="${deploy.checkout.dir}"
                            verbose="true" />                                     
</target>

</project>