Horizontally mirror a brainflak program
Charcoal, 3 bytes

Ｓ‖Ｔ


Try it online! Link is to verbose version of code. Explanation: Ｓ reads the input and implicitly echos it, while ‖ is the mirroring operator, which normally just reverses the input, but the Ｔ modifies it to mirror the characters at the same time.