impl unchecking (manually and by adding subtask)

todo.pl

@@ -40,6 +40,7 @@ new(X,Y) :- idx(Y, Task), new(X, Task).
 new(X,Y) :- 
     asserta(task(X, todo)), 
     asserta(requires(X, Y)),
+    uncheck(X), 
     write("added task "), 
     write(X),
     write(" as a subtask for "), 
@@ -52,10 +53,11 @@ depend(X,Y) :-
     asserta(requires(X, Y)).
 depend(X,Y) :- 
     task(X, done),
-    task(Y, _),
-    retractall(task(Y, _)),
-    asserta(task(Y, todo)),
-    asserta(requires(X, Y)).
+    task(Y, todo),
+    uncheck(X).
+depend(X,Y) :- 
+    task(X, done),
+    task(Y, done).
 
 check(X) :- idx(X, Task), check(Task).
 check(X) :- 
@@ -75,11 +77,33 @@ check(X) :-
     write(" before doing "),
     write(X), 
     !.
+check(X) :- 
+    task(X, done),
+    write("the task is already done "),
+    !.
 check(X) :- 
     \+ task(X, todo),
     write("no such task as "),
     write(X).
 
+uncheck(X) :- idx(X, Task), uncheck(Task).
+uncheck(X):-
+    task(X, done),
+    retractall(task(X, done)),
+    asserta(task(X, todo)), 
+    write("unchecked task: "),
+    write(X),
+    nl,
+    findall(Parent, requires(X, Parent), L),
+    uncheck_list(L).
+uncheck_list([]).
+uncheck_list([H|T]):-
+    task(H, todo),
+    uncheck_list(T).
+uncheck_list([H|T]):-
+    uncheck(H),
+    uncheck_list(T).
+
 save_requirement(T1, T2, Stream) :-
     requires(T1, T2),
     write_canonical(Stream, requires(T1,T2)),