@JensNRAD - i believe chris' suggestion was to break this into two inputs 
`in_topup_fieldcoef = File(exists=True, argstr="--topup=%s", requires=['in_topup_movpar'], ...)` and 
`in_topup_movpar = File(exists=True, requires=['in_topup_fieldcoef'], ...)` note the lack of argstr here.

and then overwrite `format_args`

```
if name == 'in_topup_fieldcoeff`:
    return spec.argstr % value.split('_fieldcoef')[0]
```