code: def pauli_from_char(ch, n=0):
      
      ch = ch.upper()
      if ch == "I":
          return I
      if ch == "X":
          return X(n)
      if ch == "Y":
          return Y(n)
      if ch == "Z":
          return Z(n)
      raise ValueError("ch shall be X, Y, Z or I")
docs: Make Pauli matrix from an character.
  
      Args:
          ch (str): "X" or "Y" or "Z" or "I".
          n (int, optional): Make Pauli matrix as n-th qubits.
  
      Returns:
          If ch is "X" => X, "Y" => Y, "Z" => Z, "I" => I
  
      Raises:
          ValueError: When ch is not "X", "Y", "Z" nor "I".