docs: def register(self, new_formulas, *args, **kwargs):
          """
          Register formula and meta data.
  
          * ``islinear`` - ``True`` if formula is linear, ``False`` if non-linear.
          * ``args`` - position of arguments
          * ``units`` - units of returns and arguments as pair of tuples
          * ``isconstant`` - constant arguments not included in covariance
  
          :param new_formulas: new formulas to add to registry.
          """
          kwargs.update(zip(self.meta_names, args))
          # call super method, meta must be passed as kwargs!
          super(FormulaRegistry, self).register(new_formulas, **kwargs)