def find(self, type_=None, f=None):
        """
        Iterates over the pool, yielding each matching ``Constant``. Calling
        without any arguments is equivalent to iterating over the pool.

        :param type_: Any subclass of :class:`Constant` or ``None``.
        :param f: Any callable which takes one argument (the constant).
        """
        for constant in self:
            if type_ is not None and not isinstance(constant, type_):
                continue

            if f is not None and not f(constant):
                continue

            yield constant
Iterates over the pool, yielding each matching ``Constant``. Calling
        without any arguments is equivalent to iterating over the pool.

        :param type_: Any subclass of :class:`Constant` or ``None``.
        :param f: Any callable which takes one argument (the constant).