class StringFunction_v4:
    def __init__(self, expression, **kwargs):
        self._f_compiled = compile(expression, 
                                   '<string>', 'eval')
        self._var = kwargs.get('independent_variable', 'x')
        self._prms = kwargs
	try:    del self._prms['independent_variable']
        except: pass

    def set_parameters(self, **kwargs):
        self._prms.update(kwargs)

    def __call__(self, x):
        self._prms[self._var] = x
        return eval(self._f_compiled, globals(), self._prms)