docs: def frequencies_iter(self):
          """
          Iterates over the frequencies of all clamped variables
  
          Yields
          ------
          tuple[ caspo.core.literal.Literal, float ]
              The next tuple of the form (literal, frequency)
          """
          df = self.to_dataframe()
          n = float(len(self))
          for var, sign in it.product(df.columns, [-1, 1]):
              f = len(df[df[var] == sign]) / n
              if f > 0:
                  yield Literal(var, sign), f