Benchmark: timing 5 iterations of int_iterate, iterate, lex_iterate...
int_iterate: 14 wallclock secs (14.19 usr +  0.12 sys = 14.31 CPU) @  0.35/s (n=5)
   iterate: 53 wallclock secs (52.45 usr +  0.15 sys = 52.60 CPU) @  0.10/s (n=5)
lex_iterate: 47 wallclock secs (47.03 usr +  0.11 sys = 47.14 CPU) @  0.11/s (n=5)
            s/iter     iterate lex_iterate int_iterate
iterate       10.5          --        -10%        -73%
lex_iterate   9.43         12%          --        -70%
int_iterate   2.86        268%        229%          --