Added Either[A, B] in the leon Library

library/lang/Either.scala

+/* Copyright 2009-2015 EPFL, Lausanne */
+
+package leon.lang
+
+import leon.annotation._
+
+/**
+ * @author Viktor
+ */
+@library
+sealed abstract class Either[A,B] {
+  def isLeft : Boolean
+  def isRight : Boolean
+  def swap : Either[B,A]
+}
+@library
+case class Left[A,B](content: A) extends Either[A,B] {
+  def isLeft = true
+  def isRight = false
+  def swap = Right[B,A](content)
+}
+@library
+case class Right[A,B](content: B) extends Either[A,B] {
+  def isLeft = false
+  def isRight = true
+  def swap = Left[B,A](content)
+}
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