additional regression test for unapply patterns

src/test/resources/regression/verification/isabelle/valid/Unapply.scala

+import leon.annotation._
+import leon.collection._
+import leon.lang._
+
+object Unapply {
+
+  @isabelle.proof(method = """(cases "<var xs>", auto)""")
+  def check[A](xs: List[A]) = {
+    xs match {
+      case Nil() => true
+      case _ :: _ => true
+    }
+  }.holds
+
+}