Benchmark:
timing 1000000 iterations of
 a_push, b_pop
...

    a_push:  4 wallclock secs ( 3.31 usr +  0.08 sys =  3.39 CPU) @ 294898.26/s (n=1000000)

     b_pop:  4 wallclock secs ( 3.91 usr +  0.00 sys =  3.91 CPU) @ 256016.39/s (n=1000000)