I don't know Haskell, so I can imagine if you didn't know Python it'd be "greek to you" (unintelligible). I'll try explaining in prose what I did. [note: I also edited the grandparent comment because when I simplified the session I showed, the assignment of "s" ended up out of order]
I use the Python decimal package to calculate 17^(1/7) to 4000 decimal places. I show the first 12 characters of the ASCII representation ("1.4989198720"), then take the first 1000 characters after the decimal place. I show the first and last 10 characters of this string ("4989198720...6163659068") so that you can verify it against the string you are testing.
Then I use a list comprehension to check each 3-digit string (e.g., 001, 002, 003, ..., 999) to find out whether it is a substring of the concatenation of `s` with itself, and list the ones that don't appear. I manually abbreviated the list, but I'm saying that in my string, 000, 002, 004, 005, 009, [and some other numbers], 993, 994, 995, 998, and 999 did not appear. (The concatenation `s+s` is checked so that the subsequences that occur at the wraparound---here,684 and 849---are found)
Because some of these length-3 sequences are missing, to my understanding this does not constitute a de Bruijn sequence of order 3 on a size-10 alphabet. However, I'm also not entirely sure whether this is what you were claiming.
