What steps will reproduce the problem?
1. Add a "and" locgical check 
2. Add a "not" logical check in it
3. A wrong </condition> balise appears

Here is the package produced by wpkgexpress r12

    <package id="polices" name="polices" revision="2" priority="5"
reboot="false">
        <check type="logical" condition="and">
            <check type="file" condition="exists"
path="%WINDIR%\Fonts\STIXNonUniBolIta.otf" />
            <check type="file" condition="exists"
path="%WINDIR%\Fonts\LinLibertine_BI-4.0.5.ttf" />
            <check type="logical" condition="not">
                <check type="file" condition="exists"
path="%windir%\fonts\cmsy10.ttf" />

            </condition>
        </check>
        <install cmd='%COMSPEC% /c copy /Y "%SOFTWARE%\polices\*.*"
"%WINDIR%\Fonts\" ' timeout="300" />
        <install cmd="%comspec% /c del  /Q /S /F %windir%\fonts\cmex10.ttf
%windir%\fonts\cmmi10.ttf %windir%\fonts\cmr10.ttf
%windir%\fonts\cmsy10.ttf" timeout="60" />
        <upgrade cmd='%COMSPEC% /c copy /Y "%SOFTWARE%\polices\*.*"
"%WINDIR%\Fonts\" ' timeout="300" />
        <upgrade cmd="%comspec% /c del /Q /S /F %windir%\fonts\cmex10.ttf
%windir%\fonts\cmmi10.ttf %windir%\fonts\cmr10.ttf
%windir%\fonts\cmsy10.ttf" timeout="60" />
    </package>


Checks are well formatted in the UI, but xml is false