(Code shortening)
Suppose a binary code of length n has M words and a minimum distance d-an (n, M, d) code. The set can be shortened by using only the words that agree in a certain position.
For example, one may consider all the words in the code that have 0 in the second position. Taking this subset of words and dropping the selected position produces a set of words of length n-1 but still with minimum distance no greater than d.
Produce a set of eight words of length six by applying this technique to the first position of the Hamming [7, 4] code.