docs: def addModification(self, aa,position, modMass, modType):
          """
          !!!!MODIFICATION POSITION IS 0 BASED!!!!!!
          Modifications are stored internally as a tuple with this format:
          (amino acid modified, index in peptide of amino acid, modification type, modification mass)
          ie (M, 7, Oxidation, 15.9...)
          such as: M35(o) for an oxidized methionine at residue 35
          """
          #clean up xtandem
          if not modType:
              #try to figure out what it is
              tmass = abs(modMass)
              smass = str(tmass)
              prec = len(str(tmass-int(tmass)))-2
              precFormat = '%'+'0.%df'%prec
              # modType = ""
              # masses = config.MODIFICATION_MASSES
              # for i in masses:
              #     if tmass in masses[i] or smass == precFormat%masses[i][0]:
              #         #found it
              #         modType = i
              # if not modType:
              #     sys.stderr.write('mod not found %s\n'%modMass)
          self.mods.add((aa,str(position),str(modMass),str(modType)))