Try to make union example...

Hello, everybody!
Not so far I used MASM to write programs in assembler language, but when I found FASM, I have seen some of its advantages and now I study this assembler language. I likes it for its simplicity an because of opportunity to write everything I need by myself.
Now let me show the result of my study. I have read some useful topics of this forum and try to combine what I have read into some useful thing.
Please, don't be very critic to me.

Now, union macro:

Code:

; macro for declaring a union

macro union _name,[def] 
{ 
 common size@union = 0 
         origin@union = $ 
 forward 
  virtual 
   #_name#def 
   if $-origin@union > size@union 
    size@union = $-origin@union 
   end if 
  end virtual 
 common
 local ..array
 struc _name {
..array  rb size@union
 forward
  virtual at ..array
   def
  end virtual
 common
 uend  
}

uend fix }


;********   example of usage   **********

; declaring a union

union MYUNION,\
       .value1             dd              ?     ,\
       .value2             db              ?     ,\
       .struct1            SOMESTRUCT            ,\
       .value3             rb              100     

; defining variable

Union1 MYUNION

; acessing fields

 mov eax,[Union1.value1]
 mov al,[Union1.value2]
 mov eax,[Union1.struct1.some_struct_field]
; ... and so on

; it is possible to write in this way:

struc COMBINED
{
 .a     dd   ?
 .b     MYUNION
 .c     db   ?
}

; or to create unions of structures:

union MYUNION,\
       .value1             SOMESTRUCT1                   ,\
       .value2             SOMESTRUCT2

If it isn't hard for you, please test this macro and tell me about my mistakes.