code: def _norm_default(x):
      
      # Lazy import to improve `import odl` time
      import scipy.linalg
  
      if _blas_is_applicable(x.data):
          nrm2 = scipy.linalg.blas.get_blas_funcs('nrm2', dtype=x.dtype)
          norm = partial(nrm2, n=native(x.size))
      else:
          norm = np.linalg.norm
      return norm(x.data.ravel())
docs: Default Euclidean norm implementation.