Write a method that accepts a string 'str' and another string 'add'. Return a copy of 'str' but with 'add' inserted between each successive letter such that the number of copies between each successive letter continues to increase by one each time. For example, if 'str' is "Summer" and 'add' is "^*" you would return "S^*u^*^*m^*^*^*m^*^*^*^*e^*^*^*^*^*r"