Benchmark: timing 10000000 iterations of _undef, empty, nothing, zero...
    _undef:  2 wallclock secs ( 0.89 usr + -0.01 sys =  0.88 CPU) @ 11363636.36/s  (n=10000000)
     empty:  2 wallclock secs ( 1.09 usr +  0.00 sys =  1.09 CPU) @ 9174311.93/s   (n=10000000)
   nothing:  0 wallclock secs ( 0.50 usr +  0.00 sys =  0.50 CPU) @ 20000000.00/s  (n=10000000)
      zero:  0 wallclock secs ( 0.06 usr +  0.00 sys =  0.06 CPU) @ 166666666.67/s (n=10000000)