Completed scaladoc in Definition.scala.
Completed scaladoc in Emptiness.scala.
Completed scaladoc in Existence.scala.
Adjusted scaladoc in Emptiness.scala.
Completed scaladoc in KeyMapping.scala.
Completed scaladoc in Length.scala.
Added scaladoc about support of arbitary object in Definition and Emptiness's companion object.
Minor scaladoc adjustments in Containing.scala.