def concat_shift_indices(n, shift):
  """Returns the concat expression of the bits at shift (shift).

  Args:
    n: An integer, the number of bits in the bit-string
    shift: An integer, the nested shift we're at.
  """
  concats = [f"shl_{shift}_0"]
  for i in range(1, n):
    rhs = concats[i - 1]
    concat = ["(concat", f"shl_{shift}_{i}", rhs + ")"]
    concats.append(" ".join(concat))
  return concats[-1]