input: Write a function to find all substrings of length k or more from a given string.
  assert find_all_substrings("abcdefg", 2) == ['ab', 'abc', 'abcd', 'abcde', 'abcdef', 'abcdefg', 'bc', 'bcd', 'bcde', 'bcdef', 'bcdefg', 'cd', 'cde', 'cdef', 'cdefg', 'de', 'def', 'defg', 'ef', 'efg', 'fg']
  assert find_all_substrings("hello", 3) == ['hel', 'hell', 'hello', 'ell', 'ello', 'llo']
  assert find_all_substrings("test", 1) == ['t', 'te', 'tes', 'test', 'e', 'es', 'est', 's', 'st', 't']
output: def find_all_substrings(s, k):
      substrings = []
      n = len(s)
      for i in range(n):
          for j in range(i + k, n + 1):
              substrings.append(s[i:j])
      return substrings