Jumping Out Of The System...
There are three letters in this puzzle: M, I, U. Initial given string is MI and from this string, other strings can be produced using the given four rules. These rules can be applied any number of times and in any order. Now, can you produce MU?* Don't worry if you don't get it. Just try it out a bit — the main thing is to get the flavor of it. Have fun 😀 (Please try it before continuing...)