define_subst question

[Andrew Stubbs <ams at codesourcery dot com>]

I want to use define_subst like this:

(define_subst "vec_merge_with_vcc"
  [(set (match_operand 0)
        (match_operand 1))
   (set (match_operand 2)
        (match_operand 3))]
  ""
  [(parallel
     [(set (match_dup 0)
           (vec_merge
             (match_dup 1)
             (match_operand 4 "" "0")
             (match_operand 5 "" "e")))
      (set (match_dup 2)
           (and (match_dup 3)
                (match_operand 5 "" "e")))])])

(Predicates and modes removed for brevity.)

This works perfectly except for operand 5. :-(

The problem is that I can't find a way to make them "match". As it is 
the substitution creates two operands, with different numbers, which 
obviously is not what I want.

The manual says that I can't do (match_dup 5), and indeed trying that 
results in a segmentation fault.

The "e" register class actually contains only one register, so I tried 
using (reg:DI EXEC_REG) for the second instance, but that caused an ICE 
elsewhere.

I could use an unspec to make the second reference implicit, or just lie 
to the compiler and pretend it's not there, but ....

How can I do this properly, please?

Thanks

Andrew