define val mov ax,[reg+si]

macro use x
{
  local new, is_set
  new equ
  irps v, val \{
    is_set equ 0
    match =reg, v \\{
      new equ new x
      is_set equ 1
    \\}
    match =0, is_set \\{
      new equ new v
    \\}
  \}
  new
}

use bx    

define val mov ax,[reg+si]

macro replace_and_use x, y, val&
{
  local result, is_set
  result equ
  match val, val \{
    irps v, val \\{
      is_set equ 0
      match =y, v \\\{
        result equ result x
        is_set equ 1
      \\\}
      match =0, is_set \\\{
        result equ result v
      \\\}
    \\}
  \}
  result
}

replace_and_use reg, bx, val