Express given key's value as a boolean type.

        Typically, this is used for ``ssh_config``'s pseudo-boolean values
        which are either ``"yes"`` or ``"no"``. In such cases, ``"yes"`` yields
        ``True`` and any other value becomes ``False``.

        .. note::
            If (for whatever reason) the stored value is already boolean in
            nature, it's simply returned.

        .. versionadded:: 2.5

def as_bool(self, key):
        """
        Express given key's value as a boolean type.

        Typically, this is used for ``ssh_config``'s pseudo-boolean values
        which are either ``"yes"`` or ``"no"``. In such cases, ``"yes"`` yields
        ``True`` and any other value becomes ``False``.

        .. note::
            If (for whatever reason) the stored value is already boolean in
            nature, it's simply returned.

        .. versionadded:: 2.5
        """
        val = self[key]
        if isinstance(val, bool):
            return val
        return val.lower() == "yes"