Optional instruction arguments -> no match

[jmerdich]

About half the instructions in the DSP56k can take an optional 'parallel move' directive which is otherwise just padding. Since this applies to so many instructions, I'd like to use a subrule that can evaluate to nothing, but it doesn't seem like the existing 'optional' matcher can match in that case. I'm open to expressing this some other way if this isn't idiomatic.

#subruledef acc {
    a => 0`1
    b => 1`1
}
#subruledef paropts {
    {} => 0`16  ; Optional and not provided, all fields are 0.
    FOO => 1`16 ; Demonstrate having an arg works, not actual syntax
}
#ruledef {
    nop =>  0`24
    abs {reg: acc} {p : paropts} => 2`4 @ reg @ 6`3 @ p
}
nop
abs a FOO
abs a

Provides:

customasm v0.13.7 (x86_64-pc-windows-msvc)
assembling `.\dsp56.asm`...
error: no match found for instruction
  --> .\dsp56.asm:15:1:
13 | nop  
14 | abs a FOO
15 | abs a
   | ^^^^^   
16 |   

As an aside, having optional arguments for an instruction is relatively common for instruction sets (judging by the admittedly niche ones I use at $DAYJOB), where they can be specified in any order in the assembler but is always in the same place in the instruction and the absence of the flag just evaluates to a default value 0. Even with the above fixed, I'd still need separate subrules for each order. For the DSP56k, it's small enough that I can just add extra subrules for each permutation but is there a good way to handle this cleanly?