Given a string of digits, output all the contiguous substrings of length
that string in the order that they appear.
For example, the string "49142" has the following 3-digit series:
And the following 4-digit series:
And if you ask for a 6-digit series from a 5-digit string, you deserve whatever you get.
Note that these series are only required to occupy adjacent positions in the input; the digits need not be numerically consecutive.
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