code: def get_precursor_mz(exact_mass, precursor_type):
      
  
      # these are just taken from what was present in the massbank .msp file for those missing the exact mass
      d = {'[M-H]-': -1.007276,
           '[M+H]+': 1.007276,
           '[M+H-H2O]+': 1.007276 - ((1.007276 * 2) + 15.9949)
           }
  
      try:
  
          return exact_mass + d[precursor_type]
      except KeyError as e:
          print(e)
          return False
docs: Calculate precursor mz based on exact mass and precursor type
  
      Args:
          exact_mass (float): exact mass of compound of interest
          precursor_type (str): Precursor type (currently only works with '[M-H]-', '[M+H]+' and '[M+H-H2O]+'
  
      Return:
            neutral mass of compound