This is the implementation of Algorithm t_{l}.
Input: string x and set U = {u_{1},…,
u_{m}}
of strings where
0 <  u_{1}
= … =  u_{m} ≤ x
Output: the minimum number t such that U is a set of approximate
 u_{1}covers
for x
with levenshtein distance t.
NOTE:
 A legal string is based on the English alphabet, no spaces are allowed.
 In this program, we use the symbol "~" to separate elements of set U.
