adding element to collection in applicationHost.config with Powershell WebAdministration module

how to add a <clear /> element and <remove attributeName=attributeValue /> to the collection with powershell webadministration module?

I can add something to collection with this command:

Add-WebConfiguration "/system.ftpServer/security/ipSecurity" -Value @{ipaddress='1.2.3.4';subnetMask='255.255.255.0'}

the result:

    <system.ftpServer>
        <security>
            <authorization />
            <ipSecurity>
                <add ipAddress="1.2.3.4" subnetMask="255.255.255.0" />
            </ipSecurity>
        </security>
    </system.ftpServer>

but I have no idea how to add <clear /> and <remove /> ellements... please help!