Constant forest_actor::miner::WPOST_CHALLENGE_LOOKBACK

pub const WPOST_CHALLENGE_LOOKBACK: ChainEpoch = 20;

Lookback from the deadline’s challenge window opening from which to sample chain randomness for the challenge seed. This lookback exists so that deadline windows can be non-overlapping (which make the programming simpler) but without making the miner wait for chain stability before being able to start on PoSt computation. The challenge is available this many epochs before the window is actually open to receiving a PoSt.