code: def sequence_molar_extinction_280(seq):
      
      if 'X' in seq:
          warnings.warn(_nc_warning_str, NoncanonicalWarning)
      return sum([residue_ext_280[aa] * n for aa, n in Counter(seq).items()])
docs: Returns the molar extinction coefficient of the sequence at 280 nm.
  
      Notes
      -----
      Units = M/cm
  
      Parameters
      ----------
      seq : str
          Sequence of amino acids.