diff --git a/ex-4_61-xx.scm b/ex-4_61-xx.scm
index 898844f..7930ba5 100644
--- a/ex-4_61-xx.scm
+++ b/ex-4_61-xx.scm
@@ -49,13 +49,6 @@
 
 (display "\nex-4.63 - genesis-family\n")
 
-; Formulate rules such as ``If S is the son of F, and F is the son of G, then S
-; is the grandson of G'' and ``If W is the wife of M, and S is the son of W,
-; then S is the son of M'' (which was supposedly more true in biblical times
-; than today) that will enable the query system to find the grandson of Cain;
-; the sons of Lamech; the grandsons of Methushael. (See exercise 4.69 for some
-; rules to deduce more complicated relationships.)
-
 (eval-query '(rule (grandson ?grandparent ?grandson)
                    (and (nson ?grandparent ?son)
                         (nson ?son ?grandson))))
@@ -70,7 +63,83 @@
 (eval-query '(grandson Methushael ?x))
 (newline)
 
-(display "\nex-4.64\n")
+(display "\nex-4.64 - outranked-loop\n")
 
-(display "\nex-4.65\n")
+(eval-query
+ '(rule (outranked-by-2 ?staff-person ?boss)
+        (or (supervisor ?staff-person ?boss)
+            (and (outranked-by-2 ?middle-manager ?boss)
+                 (supervisor ?staff-person ?middle-manager)))))
+
+; The first part of the or-clause finds a single stream. When evaluating the
+; second part of the and clause ?middle-manager and ?boss are not assigned to
+; any values yet. Therefore, in the recursive application of the rule
+; outranked-by-2 is invoked again. Again, with no assignments. By swapping the
+; two patterns in the and-clause the rule first creates frames and then invokes
+; the recursive call. If supervisor does not return any results outranked-by-2
+; is not invoked again and there is no endless-loop.
+
+; (eval-query '(outranked-by-2 (Bitdiddle Ben) ?who))
+(display "[answered]\n")
+
+(display "\nex-4.65 - four-wheels\n")
+
+;(rule (wheel ?person)
+;      (and (supervisor ?middle-manager ?person)
+;           (supervisor ?staff ?middle-manager)))
+
+; The wheel rule returns a match for each staff person that is under a
+; middle-manager. Ben and Eben are under Oliver. Ben supervises  Alyssa, Cy D,
+; and Lem E. Eben supervises Robert. Hence, there are four results for Oliver.
+; Louis works under Alyssa who works under Ben, which is why Ben only shows up
+; once.
+
+(eval-query '(wheel ?who))
+(display "\n[answered]\n")
+
+(display "\nex-4.66 - accumulation-function\n")
+
+; Ben's solution would yield wrong results for a query like wheel where the
+; same entry is returned multiple times. For the example provided, everything
+; would work fine. So, I am not sure if this is what Ben has realized. A way to
+; salvage the situation would be to apply a filter so that each ?x only occurs
+; once.
+
+(display "[answered]\n")
+
+(display "\nex-4.67 - avoid-loops\n")
+
+; Each application of a query generates a message that contains the rule name,
+; the variables, and the variable bindings. Before we apply message we check if
+; the message is in the message list in which case we skip the evaluation.
+; Otherwise, we add message to the list and continue the evaluation.
+
+(display "[answered]\n")
+
+(display "\nex-4.68 - reverse\n")
+
+(eval-query '(rule (reverse () ())))
+
+(eval-query
+  '(rule (reverse (?x . ?xs) ?rs)
+         (and (reverse ?xs ?ys)
+              (append-to-form ?ys (?x) ?rs))))
+
+(eval-query '(reverse (1 2 3) ?x))
+(newline)
+; (eval-query '(reverse ?x (1 2 3))) ; infinite loop
+
+(display "\nex-4.69 - greats\n")
+
+; Beginning with the data base and the rules you formulated in exercise 4.63,
+; devise a rule for adding ``greats'' to a grandson relationship. This should
+; enable the system to deduce that Irad is the great-grandson of Adam, or that
+; Jabal and Jubal are the great-great-great-great-great-grandsons of Adam.
+; (Hint: Represent the fact about Irad, for example, as ((great grandson) Adam
+; Irad). Write rules that determine if a list ends in the word grandson. Use
+; this to express a rule that allows one to derive the relationship ((great .
+; ?rel) ?x ?y), where ?rel is a list ending in grandson.) Check your rules on
+; queries such as ((great grandson) ?g ?ggs) and (?relationship Adam Irad).
+
+(display "\nex-4.70\n")