Scala の assert メソッドは、引数の Boolean をチェックし、値が false ならば
AssertionError を投げる。渡された Boolean が true なら、そのまま何もせずに戻ってくる。

java.lang.AssertionError: assertion failed
        at scala.Predef$.assert(Predef.scala:79)
        at Main$$anon$1.((virtual file):27)
        at Main$.main((virtual file):5)
        at Main.main((virtual file))
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at scala.tools.nsc.util.ScalaClassLoader$$anonfun$run$1.apply(ScalaClassLoader.scala:81)
        at scala.tools.nsc.util.ScalaClassLoader$class.asContext(ScalaClassLoader.scala:24)
        at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:86)
        at scala.tools.nsc.util.ScalaClassLoader$class.run(ScalaClassLoader.scala:81)
        at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:86)
        at scala.tools.nsc.ScriptRunner$.scala$tools$nsc$ScriptRunner$$runCompiled(ScriptRunner.scala:298)
        at scala.tools.nsc.ScriptRunner$$anonfun$runScript$1.apply(ScriptRunner.scala:325)
        at scala.tools.nsc.ScriptRunner$$anonfun$runScript$1.apply(ScriptRunner.scala:325)
        at scala.tools.nsc.ScriptRunner$$anonfun$withCompiledScript$2.apply(ScriptRunner.scala:282)
        at scala.tools.nsc.ScriptRunner$$anonfun$withCompiledScript$2.apply(ScriptRunner.scala:282)
        at scala.Option.map(Option.scala:75)
        at scala.tools.nsc.ScriptRunner$.withCompiledScript(ScriptRunner.scala:282)
        at scala.tools.nsc.ScriptRunner$.runScript(ScriptRunner.scala:325)
        at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:86)
        at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)