<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def crossrefs(self):
 """Returns a set of non-local targets referenced by this build file."""

        # TODO: memoize this?
        crefs = set()
        for node in self.node:
            if node.repo != self.target.repo or node.path != self.target.path:
                crefs.add(node)
        return crefs