A double occurrence word is a string w over an alphabet Ein which each symbol in either does not appear in w or appears exactly twice in w. For instance, over the ternary alphabet {0, 1, 2}, the words 011022, 1212, and 00 are double occurrence words, but 0, 0012, and 001011 are not. How many double occurrence words of length 6 over an alphabet of size 7 are there