  We consider the problem of identifying tandem scattered subsequences within a
string. Our algorithm identifies a longest subsequence which occurs twice
without overlap in a string. This algorithm is based on the Hunt-Szymanski
algorithm, therefore its performance improves if the string is not self
similar. This occurs naturally on strings over large alphabets. Our algorithm
relies on new results for data structures that support dynamic longest
increasing sub-sequences. In the process we also obtain improved algorithms for
the decremental string comparison problem.
