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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def visit_Placeholder(self, pattern):
 """ Save matching node or compare it with the existing one. FIXME : What if the new placeholder is a better choice? """

        if (pattern.id in self.placeholders and
                not Check(self.node, self.placeholders).visit(
                    self.placeholders[pattern.id])):
            return False
        else:
            self.placeholders[pattern.id] = self.node
            return True