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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 shapeExprFor(self, id_: Union[ShExJ.shapeExprLabel, START]) -> Optional[ShExJ.shapeExpr]: """ Return the shape expression that corresponds to id """

        rval = self.schema.start if id_ is START else self.schema_id_map.get(str(id_))
        return rval