// use the built-in ios linguistics functionality to stem the tags
+ (NSMutableArray *)stemTags:(NSMutableArray*)originalTags {
    
    NSLinguisticTagger *tagger = [[NSLinguisticTagger alloc]
                                  initWithTagSchemes:[NSArray arrayWithObjects:NSLinguisticTagSchemeLemma, nil]
                                  options:(NSLinguisticTaggerOmitWhitespace | NSLinguisticTaggerOmitPunctuation)];
    NSMutableArray *stemmedTags = [[NSMutableArray alloc] init];

    // convert tags to string
    [tagger setString:[originalTags componentsJoinedByString:@" "]];
    __block int i=0;
    // loop through each tag and stem it, documentation found here:
    // http://developer.apple.com/library/ios/#documentation/cocoa/reference/NSLinguisticTagger_Class/Reference/Reference.html
    [tagger enumerateTagsInRange:NSMakeRange(0, allTags.length)
                          scheme:NSLinguisticTagSchemeLemma
                         options:(NSLinguisticTaggerOmitWhitespace | NSLinguisticTaggerOmitPunctuation)
                      usingBlock:^(NSString *tag, NSRange tokenRange, NSRange sentenceRange, BOOL *stop) {
                          // tag has been stemmed, add the stemmed version to the list
                          if(tag) {
                              [stemmedTags addObject:tag];
                          }
                          // tag was not stemmed, add the original version to the list
                          else {
                              if(originalTags.count > i) {
                                  [stemmedTags addObject:[originalTags objectAtIndex:i]];
                              }
                          }
                          i++;
                      }];
    return stemmedTags;
}