User blog comment:LittlePeng9/Random Turing machines/@comment-5529393-20130331120848

I'm curious - your two-color restricted Knuth notation program will have an input of alternating 1's and _'s. How does your program tell when it gets to the end of the input string? In your original Knuth notation program, the input string was x's and ^'s, so when you reached a _ you knew you were outside the input string. But with two colors you can't tell.