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trait Token[ValueType] {
    def value: ValueType
 
    def stringValue: String = value.toString
 
    override def toString: String = stringValue
}
 
// ----
 
trait BaseToken[ValueType] extends Token[ValueType]
 
// ----
 
abstract sealed case class Keyword(name: String) extends BaseToken[Keyword] {
    override def value: Keyword = this
 
    override def toString = name
}
 
case object AND extends Keyword("AND")

/opt/scala/bin/scalac: line 50: /dev/null: Permission denied
Main.scala:21: error: case object AND has case ancestor Keyword, but case-to-case inheritance is prohibited. To overcome this limitation, use extractors to pattern match on non-leaf nodes.
case object AND extends Keyword("AND")
            ^
one error found
spoj: The program compiled successfully, but Main.class was not found.
      Class Main should contain method: def main(args: Array[String]).

Standard output is empty