Add tasks for all the new languages

Task/Stack/XLISP/stack-1.xlisp

@@ -0,0 +1,17 @@
+(define-class stack
+    (instance-variables vals))
+
+(define-method (stack 'initialize)
+    (setq vals '())
+    self)
+
+(define-method (stack 'push x)
+    (setq vals (cons x vals)))
+
+(define-method (stack 'pop)
+    (define tos (car vals))
+    (setq vals (cdr vals))
+    tos)
+
+(define-method (stack 'emptyp)
+    (null vals))