-  (re)indent an XML file
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% setenv XMLLINT_INDENT "    "
% xmllint --format oopack.xml > blah.xml
 -  Reading property lists from XML
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This will read property lists in XML (or any of the other property list formats:)
    NSData *data = // get NSData from somewhere, like NSFileManager
    if (data) {
        myRootObject = [NSPropertyListSerialization
                           propertyListFromData: data
                           mutabilityOption: NSPropertyListMutableContainers
                           format: nil
                           errorDescription: nil];
    }
For the mutability options of the resulting object, you can also use NSPropertyListImmutable and NSPropertyListMutableContainersAndLeaves -  Saving property lists as XML
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You can save any of the "property list" classes (NSDictionary, NSArray,NSNumber, NSString, NSData) as XML like this:
    NSData *data;
    data = [NSPropertyListSerialization
               dataFromPropertyList: notes
               format: NSPropertyListXMLFormat_v1_0
               errorDescription: nil];
Then write out the data using NSFileManager, or whatever other mechanism you wish.