<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _pfp__watch(self, watcher):
 """Add the watcher to the list of fields that are watching this field """

        if self._pfp__parent is not None and isinstance(self._pfp__parent, Union):
            self._pfp__parent._pfp__watch(watcher)
        else:
            self._pfp__watchers.append(watcher)