docs: def inside_brain(stat_dset,atlas=None,p=0.001):
      '''calculates the percentage of voxels above a statistical threshold inside a brain mask vs. outside it
      
      if ``atlas`` is ``None``, it will try to find ``TT_N27``'''
      atlas = find_atlas(atlas)
      if atlas==None:
          return None
      mask_dset = nl.suffix(stat_dset,'_atlasfrac')
      nl.run(['3dfractionize','-template',nl.strip_subbrick(stat_dset),'-input',nl.calc([atlas],'1+step(a-100)',datum='short'),'-preserve','-clip','0.2','-prefix',mask_dset],products=mask_dset,quiet=True,stderr=None)
      s = nl.roi_stats(mask_dset,nl.thresh(stat_dset,p))
      return 100.0 * s[2]['nzvoxels'] / (s[1]['nzvoxels'] + s[2]['nzvoxels'])