Initial data commit

Task/Null-object/Lily/null-object.lily

@@ -0,0 +1,21 @@
+enum class Option[A] {
+  Some(A)
+  None
+}
+
+# Only variables of class Option can be assigned to None.
+
+# Type: Option[integer]
+var v = Some(10)
+
+# Valid: v is an Option, and any Option can be assigned to None
+v = None
+
+# Invalid! v is an Option[integer], not just a plain integer.
+v = 10
+
+# Type: integer
+var w = 10
+
+# Invalid! Likewise, w is an integer, not an Option.
+w = None