diff --git a/todo.pl b/todo.pl
index c2cb4a6..f8263de 100644
--- a/todo.pl
+++ b/todo.pl
@@ -32,15 +32,28 @@ new(X) :-
     asserta(task(X, todo)), 
     write("added task "), 
     write(X).
+check(X) :- idx(X, Task), check(Task).
 check(X) :- 
     task(X, todo), 
+    \+ (requires(Y, X), task(Y,todo)),
     asserta(task(X, done)), 
     retractall(task(X, todo)), 
     write("marked "), 
     write(X), 
-    write(" as done").
-check(X) :- idx(X, Task), check(Task).
-
+    write(" as done"),
+    !.
+check(X) :- 
+    task(X, todo), 
+    (requires(Y, X), task(Y,todo)),
+    write("you need to do "), 
+    write(Y), 
+    write(" before doing "),
+    write(X), 
+    !.
+check(X) :- 
+    \+ task(X, todo),
+    write("no such task as "),
+    write(X).
 
 save_task(Task, Stream) :-
     task(Task, State),