input: Write a python function to print all substrings of a given string.
  assert print_substrings("coding") == ["c", "co", "cod", "codi", "codin", "coding", "o", "od", "odi", "odin", "oding", "d", "di", "din", "ding", "i", "in", "ing", "n", "ng", "g"]
  assert print_substrings("abc") == ["a", "ab", "abc", "b", "bc", "c"]
  assert print_substrings("a") == ["a"]
output: def print_substrings(s):
      substrings = []
      for i in range(len(s)):
          for j in range(i+1, len(s)+1):
              substrings.append(s[i:j])
      return substrings