Add a symbol with key `name` to `scipy_data_fitting.Model.symbols`.
        Optionally, specify an alternative `string` to pass to [`sympy.Symbol`][1],
        otherwise `name` is used.

        [1]: http://docs.sympy.org/dev/modules/core.html#id4

def add_symbol(self, name, string=None):
        """
        Add a symbol with key `name` to `scipy_data_fitting.Model.symbols`.
        Optionally, specify an alternative `string` to pass to [`sympy.Symbol`][1],
        otherwise `name` is used.

        [1]: http://docs.sympy.org/dev/modules/core.html#id4
        """
        if not string: string = name
        self.symbols[name] = sympy.Symbol(string)