diff --git a/csp/search.rkt b/csp/search.rkt
index a48d6a62..758e5d7d 100644
--- a/csp/search.rkt
+++ b/csp/search.rkt
@@ -1,3 +1,44 @@
 #lang racket/base
+(require racket/class)
+
+(provide (all-defined-out))
+
+(define Problem
+  ;; The abstract class for a formal problem.  You should subclass this and
+  ;; implement the method successor, and possibly __init__, goal_test, and
+  ;; path_cost. Then you will create instances of your subclass and solve them
+  ;; with the various search functions.
+  
+  (class object%
+    (super-new)
+    
+    (init-field initial [goal null])
+    ;; The constructor specifies the initial state, and possibly a goal
+    ;; state, if there is a unique goal.  Your subclass's constructor can add
+    ;; other arguments.
+    
+    (abstract successor)
+    ;; Given a state, return a sequence of (action, state) pairs reachable
+    ;; from this state. If there are many successors, consider an iterator
+    ;; that yields the successors one at a time, rather than building them
+    ;;  all at once. Iterators will work fine within the framework.
+    
+    (define/public (goal_test state)
+      ;; Return True if the state is a goal. The default method compares the
+      ;; state to self.goal, as specified in the constructor. Implement this
+      ;; method if checking against a single self.goal is not enough.
+      (and (equal? state goal) #t))
+    
+    (define/public (path_cost c state1 action state2)
+      ;; Return the cost of a solution path that arrives at state2 from
+      ;; state1 via action, assuming cost c to get up to state1. If the problem
+      ;; is such that the path doesn't matter, this function will only look at
+      ;; state2.  If the path does matter, it will consider c and maybe state1
+      ;; and action. The default method costs 1 for every step in the path.
+      (add1 c))
+    
+    (abstract value)
+    ;; For optimization problems, each state has a value.  Hill-climbing
+    ;; and related algorithms try to maximize this value.
+    ))
 
-(provide (all-defined-out))
\ No newline at end of file