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an instance of class week4.Expr$Sum
scala.MatchError: an instance of class week4.Expr$Sum	at week4.Expr.show(ExpressionProblem.scala:8)	at week4.Expr.toString(ExpressionProblem.scala:14)	at java.lang.String.valueOf(String.java:2994)	at java.io.PrintStream.println(PrintStream.java:821)	at scala.Console$.println(Console.scala:148)	at scala.Predef$.println(Predef.scala:315)	at week4.ExpressionProblem$.main(ExpressionProblem.scala:31)	at week4.ExpressionProblem.main(ExpressionProblem.scala)