Program
Commands: > < + - . , [ ] — anything else is a comment. Careful: since . and , are commands, avoid them in comment text.
Brainfuck, except when a cell holds a prime number, + and - swap meaning. Every arithmetic step means tracking whether the current cell is prime right now.
Commands: > < + - . , [ ] — anything else is a comment. Careful: since . and , are commands, avoid them in comment text.