Export the function to a dot file
        Args:
            filename (str)

def cfg_to_dot(self, filename):
        """
            Export the function to a dot file
        Args:
            filename (str)
        """
        with open(filename, 'w', encoding='utf8') as f:
            f.write('digraph{\n')
            for node in self.nodes:
                f.write('{}[label="{}"];\n'.format(node.node_id, str(node)))
                for son in node.sons:
                    f.write('{}->{};\n'.format(node.node_id, son.node_id))

            f.write("}\n")