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https://www.myerrorsandmysolutions.com/windows-phone-sdk-update-for-7-8-warning-1-the-element-deployment-in-namespace-http-schemas-microsoft-com-windowsphone2009deployment-has-invalid-child-element-appextra-list-of-poss/
SDK update for Windows Phone 7.8 - "The element "Deployment" in namespace "http://schemas.microsoft.com/windowsphone/2009/deployment" has invalid child element "AppExtra". List of possible elements expected: "App"."