Check whether the given value exists in the given subsystem.
        Does not make a difference whether the value is readable, writable, or both.
        Do not include the subsystem name in the option name.
        Only call this method if the given subsystem is available.

def has_value(self, subsystem, option):
        """
        Check whether the given value exists in the given subsystem.
        Does not make a difference whether the value is readable, writable, or both.
        Do not include the subsystem name in the option name.
        Only call this method if the given subsystem is available.
        """
        assert subsystem in self
        return os.path.isfile(os.path.join(self.per_subsystem[subsystem], subsystem + '.' + option))