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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 add_file(self,fName,content) : """If this FSNode is a directory, write a file called fName containing content inside it"""

        if not self.isdir() : raise Exception("FSQuery tried to add a file in a node which is not a directory : %s" % self.abs)
        self.write_file("%s/%s"%(self.abs,fName),content)