Benchmark: timing 3 iterations of 20.two_string, 30.two_string_lookahead...
20.two_string: 236 wallclock secs (234.31 usr +  1.62 sys = 235.93 CPU) @  0.01/s (n=3)
30.two_string_lookahead: 178 wallclock secs (176.23 usr +  1.52 sys = 177.75 CPU) @  0.02/s (n=3)