<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def before_content(self):
 """Build up prefix history for nested elements The following keys are used in :py:attr:`self.env.ref_context`: dn:prefixes Stores the prefix history. With each nested element, we add the prefix to a list of prefixes. When we exit that object's nesting level, :py:meth:`after_content` is triggered and the prefix is removed from the end of the list. dn:prefix Current prefix. This should reflect the last element in the prefix history """

        super(DotNetObjectNested, self).before_content()
        if self.names:
            (_, prefix) = self.names.pop()
            try:
                self.env.ref_context['dn:prefixes'].append(prefix)
            except (AttributeError, KeyError):
                self.env.ref_context['dn:prefixes'] = [prefix]
            finally:
                self.env.ref_context['dn:prefix'] = prefix