missing a parenthesis here. also you might simply want to write this as:

```
pbx2 = fsl.ProbTrackX2()
pbx2.inputs.seed = 'seed_source.nii.gz'
pbx2.inputs.thsamples = 'merged_th1samples.nii.gz'
pbx2.inputs.fsamples = 'merged_f1samples.nii.gz'
pbx2.inputs.phsamples = 'merged_ph1samples.nii.gz'
pbx2.inputs.mask = 'nodif_brain_mask.nii.gz'
pbx2.inputs.out_dir = '.'
pbx2.inputs.n_samples = 3
pbx2.inputs.n_steps = 10
```

for the various files, go to `nipype/testing/data` folder and create dummy files (e.g., `touch seed_source.nii.gz`)

git add (the dummy files)

commit and push