(mod-expt-mults 8)  => 4
     (mod-expt-mults 16)  => 5
     (mod-expt-mults 15)  => 7
     (mod-expt-mults 64)  => 7
     (mod-expt-mults 63)  => 11
     (mod-expt-mults 256)  => 9
     (mod-expt-mults 257)  => 10
     (mod-expt-mults 255)  => 15