花了一些时间对一些代码进行故障排除，来到“to”的古玩bug。我错过了什么？
Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_121).
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scala> val is = collection.mutable.TreeMap[Int, Int]()
is: scala.collection.mutable.TreeMap[Int,Int] = TreeMap()
scala> is += 0 -> 9 += 70 -> 100
res0: is.type = TreeMap(0 -> 9, 70 -> 100)
scala> is.from(30)
res1: scala.collection.mutable.TreeMap[Int,Int] = TreeMap(70 -> 100)
scala> is.to(30)
res2: scala.collection.mutable.TreeMap[Int,Int] = TreeMap() // !!!!!
scala> val is2 = collection.immutable.TreeMap[Int, Int]()
is2: scala.collection.immutable.TreeMap[Int,Int] = Map()
scala> val is3 = is2 + (0 -> 9) + (70 -> 100)
is3: scala.collection.immutable.TreeMap[Int,Int] = Map(0 -> 9, 70 -> 100)
scala> is3.from(30)
res3: scala.collection.immutable.TreeMap[Int,Int] = Map(70 -> 100)
scala> is3.to(30)
**res4: scala.collection.immutable.TreeMap[Int,Int] = Map(0 -> 9)**