let add_sub_imm s insn sf =
  let%decode insn' =
"31:31:sf:F:0,30:30:op:F:0,29:29:S:F:0,28:24:_:F:10001,23:22:shift:F:xx,21:10:imm12:F:xxxxxxxxxxxx,9:5:Rn:F:xxxxx,4:0:Rd:F:xxxxx" insn
  in
  let s_b = get_s_bool insn' in
  ....