docs: def frequency(self, literal):
          """
          Returns the frequency of a clamped variable
  
          Parameters
          ----------
          literal : :class:`caspo.core.literal.Literal`
              The clamped variable
  
          Returns
          -------
          float
              The frequency of the given literal
  
          Raises
          ------
          ValueError
              If the variable is not present in any of the clampings
          """
          df = self.to_dataframe()
          if literal.variable in df.columns:
              return len(df[df[literal.variable] == literal.signature]) / float(len(self))
          else:
              raise ValueError("Variable not found: %s" % literal.variable)