<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def serialize(self, occur=None):
 """Return RELAX NG representation of the receiver and subtree. """

        fmt = self.ser_format.get(self.name, SchemaNode._default_format)
        return fmt(self, occur) % (escape(self.text) +
                                   self.serialize_children())