Add all the A tasks

Task/Abstract-type/Eiffel/abstract-type.e

@@ -0,0 +1,17 @@
+deferred class
+    AN_ABSTRACT_CLASS
+
+feature
+
+    a_deferred_feature
+        -- a feature whose implementation is left to a descendent
+        deferred
+        end
+
+    an_effective_feature: STRING
+        -- deferred (abstract) classes may still include effective features
+        do
+            Result := "I am implemented!"
+        end
+
+end