is-reg64 := λ(: s String). (: (
   (let r 0_u64)
   (match s (
      ()
      ( 'RAX_s (set r 1_u64))
      ( 'RBX_s (set r 1_u64))
      ( 'RCX_s (set r 1_u64))
      ( 'RDX_s (set r 1_u64))
      ( 'RSI_s (set r 1_u64))
      ( 'RDI_s (set r 1_u64))
      ( 'RBP_s (set r 1_u64))
      ( 'RSP_s (set r 1_u64))
      ( 'R8_s (set r 1_u64))
      ( 'R9_s (set r 1_u64))
      ( 'R10_s (set r 1_u64))
      ( 'R11_s (set r 1_u64))
      ( 'R12_s (set r 1_u64))
      ( 'R13_s (set r 1_u64))
      ( 'R14_s (set r 1_u64))
      ( 'R15_s (set r 1_u64))
      ( _ () )
   ))
   r
) U64);