code: def subat(orig, index, replace):
      
      return "".join([(orig[x] if x != index else replace) for x in range(len(orig))])
docs: Substitutes the replacement string/character at the given index in the
      given string, returns the modified string.
  
      **Examples**:
      ::
          auxly.stringy.subat("bit", 2, "n")