How to play

A loop carries out the commands inside it again and again. Select its count tile to change how many rounds it runs. A loop uses one slot for its count and one for each command inside it, and the counter above the board counts slots. While a loop runs, the count tile shows the current round.

A repeat until loop has no count. Before each round it looks at the cell straight ahead. If that cell is a wall or off the board, the loop ends. Otherwise the commands inside run once more. A loop that starts facing a wall runs none of them. It uses the same slots as any other loop, and it works for a corridor of any length.

An if block makes a decision. It has a sensor: clear ahead, clear left or clear right. The sensor is true when the cell on that side is on the board and is not a wall. Reading it costs no step. If it is true, the commands under then run. If it is not, the commands under else run, and else may be empty.

An if block uses one slot, plus one for each command in then and in else. It can stand on its own or inside a loop, but not inside the then of another if block. A repeat until loop can also wait for the goal, which keeps a program with decisions going until it arrives.

Some levels give you a counter named N. It starts at 1 each run. The count up command adds one to it and takes one slot and one step. A repeat loop can take N as its count instead of a number: it reads N when it starts and runs that many rounds, whatever happens to N inside it.

Use it for shapes that grow: a loop that walks N cells, turns, and counts up, repeated again and again, draws a spiral.

A few levels let a loop hold one more loop. The inner loop runs all its rounds each time the outer loop runs a round. That is two levels at most. Slots add up: a loop uses one slot plus the slots of everything inside it, inner loop included.

The mark command leaves a breadcrumb on the cell the automaton stands on. It is a small arrow pointing to the cell behind the automaton, which is the cell it came from when it has just stepped forward. A cell keeps the first breadcrumb it gets. Marking a cell again changes nothing.

With marks on the board, an if block gains six sensors. Fresh ahead, left and right are true for an open cell with no breadcrumb. Back ahead, left and right are true for the cell that the breadcrumb under the automaton points to: the way back. Reading a sensor costs no step.

The else of an if block can hold one more if block, called else if, instead of commands. A chain of up to six ifs chooses between many cases, and the first sensor that is true wins. Each if in the chain uses one slot plus its own then commands. Together they let a program explore a maze that has loops: mark the cell, step onto a fresh cell, and follow the breadcrumbs back when none is left.

Some boards have no target. The level is solved the moment the automaton has entered or started on every floor cell, that is, every cell that is not a wall. The board shows the cells visited so far and the game counts them, for example 12 of 30 cells. A step limit still applies, so wandering fails. A common answer is to run to the wall, step over, and run back, row by row.

A procedure is a named group of commands, A, B or C. You define it once and call it from several places, so a shape that comes back along the route is written only once. A definition does not run where it stands. Each call runs the commands of that procedure and then the program goes on.

A call uses one slot. A definition uses one slot plus one for each command in it. A procedure can hold moves, turns, if blocks and loops, but not a call to another procedure. A call can stand on its own or inside a loop.

Some levels have more than one board. One program must solve every board, so a program that counts the steps of the first board fails on the next. The board area shows one board at a time with a tab for each. Run tries the program on the selected board, and a board it has solved is marked with a check and the word. Editing the program clears the checks.

• Run executes the program at a readable speed.
• Step runs one instruction at a time.
• Reset puts the automaton back at the start and keeps your program.

You can edit the program again after any run.