The Three Buttons
Frackle on the iPhone has only three buttons in its user interface. But that's all you need. The middle button displays the name of the active command. The active command controls what the other two buttons do.

Tap on the middle button to cycle through all of the available commands.

Tap on the left or the right button to execute the active command in one direction or the other.

Simple commands do not have a fewer/more or previous/next notion of direction. For a simple command, tap the left or the right button, each does the same thing.