docs: def qwarp_invert(warp_param_dset,output_dset,affine_1Dfile=None):
      '''inverts a qwarp (defined in ``warp_param_dset``) (and concatenates affine matrix ``affine_1Dfile`` if given)
      outputs the inverted warp + affine to ``output_dset``'''
  
      cmd = ['3dNwarpCat','-prefix',output_dset]
  
      if affine_1Dfile:
          cmd += ['-warp1','INV(%s)' % affine_1Dfile, '-warp2','INV(%s)' % warp_param_dset]
      else:
          cmd += ['-warp1','INV(%s)' % warp_param_dset]
  
      nl.run(cmd,products=output_dset)