def note_addition(self, key, value):
<EOL>if key in self._deleted:<EOL><INDENT>if value != self._deleted[key]:<EOL><INDENT>self._previous[key] = self._deleted[key]<EOL><DEDENT>del self._deleted[key]<EOL><DEDENT>else:<EOL><INDENT>self._added.append(key)<EOL><DEDENT>
Updates the change state to reflect the addition of a field. Detects previous
changes and deletions of the field and acts accordingly.