Benchmark: timing 1000 iterations of grep_seen, keys_map... grep_seen: 13 wallclock secs (11.17 usr + 0.00 sys = 11.17 CPU) @ 89.52/s (n=1000) keys_map: 30 wallclock secs (29.28 usr + 0.00 sys = 29.28 CPU) @ 34.15/s (n=1000)