(STRATEGY
    INNERMOST)

(VAR
    X X1 X2 X3)
(DATATYPES
    A = µX.< a, mark(X), b, ok(X) >)
(SIGNATURES
    active :: [A] -> A
    f :: [A x A x A] -> A
    proper :: [A] -> A
    top :: [A] -> A)
(RULES
    active(f(a(),X,X)) -> mark(f(X
                                ,b()
                                ,b()))
    active(b()) -> mark(a())
    active(f(X1,X2,X3)) -> f(X1
                            ,active(X2)
                            ,X3)
    f(X1,mark(X2),X3) -> mark(f(X1
                               ,X2
                               ,X3))
    proper(f(X1,X2,X3)) ->
      f(proper(X1)
       ,proper(X2)
       ,proper(X3))
    proper(a()) -> ok(a())
    proper(b()) -> ok(b())
    f(ok(X1),ok(X2),ok(X3)) ->
      ok(f(X1,X2,X3))
    top(mark(X)) -> top(proper(X))
    top(ok(X)) -> top(active(X)))