code: def add_symbol(self, name, string=None):
          
          if not string: string = name
          self.symbols[name] = sympy.Symbol(string)
docs: 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