code: def _3d_in_file(in_file):
      ''' if self.inputs.in_file is 3d, return it.
      if 4d, pick an arbitrary volume and return that.
  
      if in_file is a list of files, return an arbitrary file from
      the list, and an arbitrary volume from that file
      '''
  
      in_file = filemanip.filename_to_list(in_file)[0]
  
      try:
          in_file = nb.load(in_file)
      except AttributeError:
          in_file = in_file
  
      if in_file.get_data().ndim == 3:
          return in_file
  
      return nlimage.index_img(in_file, 0f _3d_in_file(in_file):
      ''' if self.inputs.in_file is 3d, return it.
      if 4d, pick an arbitrary volume and return that.
  
      if in_file is a list of files, return an arbitrary file from
      the list, and an arbitrary volume from that file
      '''
  
      in_file = filemanip.filename_to_list(in_file)[0]
  
      try:
          in_file = nb.load(in_file)
      except AttributeError:
          in_file = in_file
  
      if in_file.get_data().ndim == 3:
          return in_file
  
      return nlimage.index_img(in_file, 0)
docs: if self.inputs.in_file is 3d, return it.
      if 4d, pick an arbitrary volume and return that.
  
      if in_file is a list of files, return an arbitrary file from
      the list, and an arbitrary volume from that file