I think I have isolated the problem with the adding function. Once you get to an 18 digit number, instead of adding 1, it starts rounding to the nearest 17 digit number, either up or down. For example, putting in the 17 digit number “11111111111111111” gives the correct answer, “11111111111111112.” But as soon as I add another digit, it starts doing weird stuff. The result of 18 1’s (111111111111111111) is “111111111111111100”. 19 1’s gives “1111111111111111200”. So it appears that Kantu doesn’t have the capacity to handle large numbers.

See original code and question here: Variable name must start with underscore