How to use the gillespy2.FunctionDefinition function in gillespy2

def build_function_definitions(self, args):
        '''
        Build a GillesPy2 function definition.

        Attributes
        ----------
        args : dict
            A json representation of a function definition.
        '''
        name = args['name']
        variables = args['variables'].split(', ')
        expression = args['expression']

        return FunctionDefinition(name=name, args=variables, function=expression)