<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _parse_default(self):
 """Parse the `Schema` for the `Bison` instance to create the set of default values. If no defaults are specified in the `Schema`, the default dictionary will not contain anything. """

        # the configuration changes, so we invalidate the cached config
        self._full_config = None

        if self.scheme:
            self._default.update(self.scheme.build_defaults())