docs: def atlas_overlap(dset,atlas=None):
      '''aligns ``dset`` to the TT_N27 atlas and returns ``(cost,overlap)``'''
      atlas = find_atlas(atlas)
      if atlas==None:
          return None
      
      cost_func = 'crM'
      infile = os.path.abspath(dset)
      tmpdir = tempfile.mkdtemp()
      with nl.run_in(tmpdir):
          o = nl.run(['3dAllineate','-verb','-base',atlas,'-source',infile + '[0]','-NN','-final','NN','-cost',cost_func,'-nmatch','20%','-onepass','-fineblur','2','-cmass','-prefix','test.nii.gz'])
          m = re.search(r'Final\s+cost = ([\d.]+) ;',o.output)
          if m:
              cost = float(m.group(1))
          o = nl.run(['3dmaskave','-mask',atlas,'-q','test.nii.gz'],stderr=None)
          data_thresh = float(o.output) / 4
          i = nl.dset_info('test.nii.gz')
          o = nl.run(['3dmaskave','-q','-mask','SELF','-sum',nl.calc([atlas,'test.nii.gz'],'equals(step(a-10),step(b-%.2f))'%data_thresh)],stderr=None)
          overlap = 100*float(o.output) / (i.voxel_dims[0]*i.voxel_dims[1]*i.voxel_dims[2])
      try:
          shutil.rmtree(tmpdir)
      except:
          pass
      return (cost,overlap)