is-reg16 := λ(: s String). (: (
   (let r 0_u64)
   (match s (
      ()
      ( 'AX_s (set r 1_u64))
      ( 'BX_s (set r 1_u64))
      ( 'CX_s (set r 1_u64))
      ( 'DX_s (set r 1_u64))
      ( 'SI_s (set r 1_u64))
      ( 'DI_s (set r 1_u64))
      ( 'BP_s (set r 1_u64))
      ( 'SP_s (set r 1_u64))
      ( 'R8W_s (set r 1_u64))
      ( 'R9W_s (set r 1_u64))
      ( 'R10W_s (set r 1_u64))
      ( 'R11W_s (set r 1_u64))
      ( 'R12W_s (set r 1_u64))
      ( 'R13W_s (set r 1_u64))
      ( 'R14W_s (set r 1_u64))
      ( 'R15W_s (set r 1_u64))
      ( _ () )
   ))
   r
) U64);