Each rule is write·move·next: the symbol to write, the move (L/R/N), and the next state. A blank rule means halt (stuck). Editing rebuilds the machine and resets it.
The tape is
unbounded — blanks stretch forever both ways; only the visited cells are stored. The
busy beavers are the rigorous centerpiece: from a blank tape,
BB(4) halts after exactly 107 steps
leaving 13 ones — the proven 4-state record. Try editing one of its rules and watch the proof break.
For a halting question
no one can answer, see
The Collatz Bench — run n→n/2 | 3n+1 as a program and every tested input halts at 1, but no machine can promise it always will.