(STRATEGY
    INNERMOST)

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