is-reg8 := λ(: s String). (: (
   (let r 0_u64)
   (match s (
      ()
      ( 'AL_s (set r 1_u64))
      ( 'BL_s (set r 1_u64))
      ( 'CL_s (set r 1_u64))
      ( 'DL_s (set r 1_u64))
      ( 'SIL_s (set r 1_u64))
      ( 'DIL_s (set r 1_u64))
      ( 'BPL_s (set r 1_u64))
      ( 'SPL_s (set r 1_u64))
      ( 'R8B_s (set r 1_u64))
      ( 'R9B_s (set r 1_u64))
      ( 'R10B_s (set r 1_u64))
      ( 'R11B_s (set r 1_u64))
      ( 'R12B_s (set r 1_u64))
      ( 'R13B_s (set r 1_u64))
      ( 'R14B_s (set r 1_u64))
      ( 'R15B_s (set r 1_u64))
      ( _ () )
   ))
   r
) U64);