docs: def mzminus(df, minus=0, noise=10000):
      """
      The abundances of ions which are minus below the molecular ion.
      """
      mol_ions = ((df.values > noise) * df.columns).max(axis=1) - minus
      mol_ions[np.abs(mol_ions) < 0] = 0
      d = np.abs(np.ones(df.shape) * df.columns -
                 (mol_ions[np.newaxis].T * np.ones(df.shape))) < 1
      d = (df.values * d).sum(axis=1)
      return Trace(d, df.index, name='m-' + str(minus))