# ADJUNCTION MODELS FOR CALL-BY-PUSH-VALUE WITH STACKS

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3 3 To see the benefit of this definition, fix a set R and consider the adjunction Set(X,Y R) = Set op (X R,Y ) (4) We can decompose this isomorphism quite naturally by setting O(X,Y ) to be Set(X Y,R). This is essentially what is happening in CBPV+stacks: we have three judgements, denoting C-morphisms (values), oblique morphisms (computations) and D-morphisms (stacks). But the above account is overly simplistic, for, as we shall see, we want D to be locally indexed by C Element Style vs. Naturality Style. Universal properties in category theory can usually be defined either in terms of elements or in terms of naturality of isomorphisms. Here is a well-known example. A product for a family of objects {A i } i I in a category C consists of an object V the vertex together with either of the following: for each i I, a morphism V π i A i, such that the function C(X,V ) i I C(X,A i) for all X (5) f λi.(f;π i ) is an isomorphism an isomorphism C(X,V ) = i I C(X,A i) natural in X (6) The equivalence of these two definitions follows from the Yoneda Lemma. For the universal properties we will treat in this paper, we will give definitions in element-style only, and leave the naturality-style formulation to future work see also [Lev04]. Here is an important special case, adapted from [CLW93, Coc93] and used to interpret sum types Definition. A distributive coproduct for a family of objects {A i } i I in a cartesian category C is an object V and a C-morphism A i in i V for each i I, such that the functions are isomorphisms. C(X V,Y ) i I C(X A i,y ) for all X,Y (7) f λi.((x in i );f) Clearly, any distributive coproduct is a coproduct; the converse holds if C is cartesian closed but is false in general. A cartesian category with all finite (resp. countable) distributive coproducts is called a distributive (resp. countably distributive) category.

4 2. Review of Call-By-Push-Value 4 There are two variants of CBPV: finitary and infinitely wide. In this paper we treat infinitely wide CBPV; the finitary case is treated by substituting finite for countable throughout. (The reverse substitution would not work, because for both variants contexts are finite, and hence the value category requires only finite products.) CBPV has two disjoint classes of terms: values and computations. It likewise has two disjoint classes of types: a value has a value type, while a computation has a computation type. For clarity, we underline computation types. The types are given by value types A ::= UB i I A i 1 A A computation types B ::= FA i I B i A B where I can be any countable set (finite, in finitary CBPV). The meaning of F and U is as follows. A computation of type FA returns a value of type A. A value of type UB is a thunk of a computation of type B, i.e. the computation is frozen into a value so that it can be passed around. When later required, it can be forced i.e. executed. Unlike in call-by-value, a function in CBPV is a computation, and hence a function type is a computation type. We will discuss this further in Sect. 3. Like in call-by-value, an identifier in CBPV can be bound only to a value, so it must have value type. We accordingly define a context Γ to be a sequence x 0 : A 0,...,x n 1 : A n 1 of identifiers with associated value types. We often omit the identifiers and write just A 0,...,A n 1. We write Γ v V : A to mean that V is a value of type A, and we write Γ c M : B to mean that M is a computation of type B. The terms of CBPV are given in Fig. 1. We assume formally that all terms are explicitly typed, but in this paper, to reduce clutter, we omit explicit typing information, We explain some of the less familiar constructs as follows. M to x. N is the sequenced computation that first executes M, and when, this returns a value V proceeds to execute N with x bound to V. This was written in Moggi s syntax using let, but we reserve let for mere binding. The keyword pm stands for pattern-match, and the symbol represents application in reverse order. Because we think of i I as the type of functions taking each i I to a computation of type B i, we have made its syntax similar to that of. Following Lawvere [Law63], we say that a context morphism q from Γ = A 0,...,A m 1 to = B 0,...,B n 1 is a sequence of values V 0,...,V n 1 where Γ v V i : B i. As usual, such a morphism induces (by induction [FPD99]) a substitution function q from values v V : C to values Γ v V : C and from computations c M : B to Γ c M : B. We define identity and composite context morphisms in the usual way.

5 5 Γ,x : A, Γ v x : A Γ v V : A Γ c return V : FA Γ c M : B Γ v thunk M : UB Γ v V : A Γ,x : A c M : B Γ c let V be x. M : B Γ c M : FA Γ,x : A c N : B Γ c M to x. N : B Γ v V : UB Γ c force V : B Γ v V : Aî Γ v (î,v ) : i I A i î I Γ v V : i I A i Γ,x : A i c M i : B i I Γ c pm V as {...,(i,x).m i,...} : B Γ v V : A Γ v V : A Γ v (V,V ) : A A Γ c M i : B i i I Γ c λ{...,i.m i,...} : i I B i Γ,x : A c M : B Γ c λx.m : A B Γ v V : A A Γ,x : A,y : A c M : B Γ c pm V as (x,y).m : B Γ c M : i I B i î I Γ c î M : Bî Γ v V : A Γ c M : A B Γ c V M : B Figure 1: Terms of Call-By-Push-Value

6 3. The CK-Machine and the Stack Judgement 6 The operational semantics of CBPV is given in [Lev99] in big-step form, but here we use a CK-machine, a style of semantics introduce in [FF86] for CBV there are many similar formulations [Kri85, PS98, SR98]. We describe it for computations on a fixed context Γ (by contrast with big-step semantics, which is defined for closed computations only). The machine is presented in Fig. 2, with types written explicitly. A configuration of the machine, with types, has the form Γ M B K C (8) where Γ c M : B is the computation we are currently evaluating and K is the stack (perhaps better known as an evaluation context, another concept that appeared in [FF86]). Notice that Γ and C remain fixed throughout execution. To begin with, we place the computation we wish to evaluate alongside the empty stack, and we apply transitions until we reach a terminal configuration. When the computation has the form M to x. N, we must first evaluate M, during which time we have no need of N. So we move the context [ ] to x. N onto the stack, and proceed to evaluate M. Once we have evaluated M to the form return V, we remove that context from the stack and proceed to evaluate N[V/x]. Similarly, to evaluate V M we leave the operand V on the stack while we evaluate M. Classifying a function as a computation is a novelty of CBPV and, at first sight, seems counterintuitive. But the CK-machine provides an explanation. For λx is treated as an instruction pop x whilst V is treated as an instruction push V. Thus a computation of type A B pops a value of type A and proceeds as a computation of type B. Similarly, a computation of type i I B i pops a tag i I and proceeds as a computation of type B i. In order to characterize the well-typed configurations, we require a judgement for stacks, of the form Γ B k K : C. Thus a configuration (8) will be well-typed precisely when Γ c M : B and Γ B k K : C. By inspecting Fig. 2, we can see that the typing rules for this judgment should be as given in Fig. 3. This ensures that the well-typed configurations are precisely those obtainable from an initial configuration. There are some evident operations on stacks. 1. Given a context-morphism Γ q and a stack B k K : C we can substitute q in L to give a stack Γ B k q K : C. 2. Given a computation Γ c M : B and a stack Γ B k K : C, we can dismantle K onto M to give Γ c M K : C, defined by induction on K. It is the computation whose evaluation leads to the configuration (8). Think of this operation as running the CK-machine backwards.

7 7 Initial configuration for evaluation of Γ c M : C Transitions Γ M C nil C Γ let V be x. M B K C Γ M[V/x] B K C Γ M to x. N B K C Γ M FA [ ] to x. N :: K C Γ return V FA [ ] to x. N :: K C Γ N[V/x] B K C Γ force thunk M B K C Γ M B K C Γ pm (î,v ) as {...,(i,x).m i,...} B K C Γ Mî[V/x] B K C Γ pm (V,V ) as (x,y).m B K C Γ M[V/x,V /y] B K C Γ î M Bî K C Γ M i I B i î :: K C Γ λ{...,i.m i,...} i I B i î :: K C Γ Mî Bî K C Γ V M B K C Γ M A B V :: K C Γ λx.m A B V :: K C Γ M[V/x] B K C Figure 2: CK-Machine For CBPV, With Types

8 8 Γ C k nil : C Γ Bî k K : C Γ i I B i k î :: K : C î I Γ,x : A c M : B Γ B k K : C Γ FA k [ ] to x. M :: K : C Γ v V : A Γ B k K : C Γ A B k V :: K : C Figure 3: Typing rules for stacks 3. Given two stacks Γ B k K : C and Γ C k L : D, we can concatenate 1 K and L to give Γ B k K++L : D. We can regard concatenation as a form of substitution, because K +L is K[L/nil] Lemma. Substitution, dismantling and concatenation satisfy the following properties. nil +K = K p nil = nil K +nil = K p (K +L) = (p K) +(p L) (K +L) +L = K +(L +L ) p (M K) = (p M) (p K) M nil = M M (K +K ) = (M K) K id P = P (p;q) P = p q P 4. Basic Structure 4.1. Interpreting Values. Like Moggi, we interpret values in a cartesian category C, called the value category. As usual, the products in C are used for interpreting both and context extension (comma). Although there is no value in the language of the form x : A A v V : A (for general A, A ), there will be after we extend the syntax in Sect Interpreting Stacks. If values are interpreted in the cartesian category C, stacks will be interpreted in a locally C-indexed category D. This can be defined in various ways: as a strict C-indexed category (i.e. functor C op Cat) in which all the fibres D X have the same set of objects ob D and all the reindexing functors D f are identityon-objects as a [C op,set]-enriched category using the following concrete description. 1 The reader who finds the idea of concatenating stacks to be strange need not worry, because concatenation and dismantling are never performed in the operational semantics. It is only for reasoning about stacks that we consider these operations.

9 Definition. A locally C-indexed category consists of a set ob D of D-objects which we underline (except where ob D = ob C) for each object X ob C and each pair of objects Y,Z ob D, a small set D X (Y,Z) of D-morphisms written Y f Z X for each object X ob C and each object Y ob D, an identity morphism Y id X,Y X Y for each morphism Y W, a composite morphism Y f;g W X f X Z and each morphism Z g X for each D-morphism Y D-morphism Y such that k f Z X f X Z and each C-morphism X k X, a reindexed id;f = f k id = id id f = f f; id = f k (f;g) = (k f); (k g) (l;k) f = l (k f) (f;g);h = f; (g;h) It is easy to see that this is natural for interpreting stacks: a computation type will denote an object of D and a stack Γ B k K : C will denote a D-morphism over [Γ] from [B] to [C ]. Then identity morphisms in D interpret nil, composition interprets concatenation of stacks, and reindexing interprets substitution. Before proceeding further, we develop some theory of locally indexed categories. Firstly, it is clear that they form a 2-category, and we have operations op and. The most important example of a locally C-indexed category is called self C, and given by ob self C = ob C self C A (B,C) = C(A B,C) with the evident composition and reindexing. This is used in the following result, mentioned in [Mog91] Proposition. A strong monad on C corresponds to a monad on self C. As with ordinary categories, we wish to speak of the homset functor associated with D, and we do this using the opgrothendieck construction as follows.

10 4.5. Definition. 1. Let D be a locally C-indexed category. We write opgr D for the ordinary category where an object is a pair X Y where X ob C and Y ob D; a morphism from X Y to X Z in opgr D consists of a pair k h where X k X in C and Y h X Z in D the identity on Γ X is given by id id; 10 the composite of Γ X k f Γ Y lg Γ Z is l;k((l f);g). 2. For a locally D-indexed category, we write Hom D, or D for short, for the functor opgr(d op D) X(Y,Z) k(f,h) Set D X (Y,Z) λg.(f; (k g);h) 4.6. Interpreting Computations. If we are interpreting values in a cartesian category C, and stacks in a locally C-indexed category D, then we will interpret computations in a functor O : opgr D Set, also called a right D-module. This can be described in concrete terms Definition. A (locally C-indexed) right D-module is given by for each X ob C and Y ob D, a small set O X Y, an element of which we call an g O-morphism over X to Y and write Y X for each X k X and g X Y a reindexed O morphism k g X Y for each g X Y and Y h X Y satisfying identity, associativity and reindexing laws: a composite O morphism g; id = g id g = g k (g;h) = (k g); (k h) g; (h;h ) = (g;h);h (k;l) g = k (l g) where g is an O-morphism. Our intention is that a computation Γ c M : B will denote an O-morphism over [Γ] to [B] and that q M will denote [q] [M ] while the dismantling M K will denote [M ]; [K ]. We summarize the above discussion as follows. g;h X Y

11 Definition. A CBPV judgement model consists of a cartesian category C a locally C-indexed category D a right D-module O Examples. Here are some examples of CBPV judgement models. We do not motivate them individually, because they are given only as examples of the structure. trivial Given a cartesian category C, set D to be self C and set O X Y to be C(X,Y ). cpo Let C be Cpo (directed-complete posets and continuous maps). Let a D-object be a pointed cpo (a cpo with a least element) and a D-morphism over X from Y to Z be a right-strict continuous function from X Y to Z. Let O X Y be continuous functions from X to Y. monad Given a strong monad T on a cartesian category C, we obtain a judgement model (C, C T, O T ), where a C T -object is a T-algebra, a C T -morphism over X from (Y,θ) to (Z, φ) is a C-morphism X Y f Z satisfying X TY t(x,y ) T(X Y ) Tf TZ X θ X Y f Z φ and let OX T (Y,θ) be C(X,Y ). storage Given a set S, let C be Set, let D be self Set and let O X Y be Set(S X,Y ). erratic choice Let C be Set, let D be the locally Set-indexed category Rel in which an object is a set and a morphism over X from Y to Z is a relation from X Y to Z, and let an O-morphism over X to Y be a relation from X to Y. continuations Given a set R, let C be Set, let D be (self Set) op and let O X Y be Set(X Y,R) Example: Possible Worlds. Our next example is a possible world semantics. This is a way of modelling dynamically generated storage cells that was introduced for CBN in [Ole82] and for CBV in [Mog90]. These were models of ground storage only, but a CBV model for general storage was presented in [Lev02], and our example is the CBPV version of it. Let W be a countable category (countably many objects and morphisms), and for each w ob W let Sw be a set. (The storage model above is the special case where W is the unit category.) We define C to be [W,Set], and D to be the locally C-indexed category where

12 12 an object is a functor from W op to Set a morphism over X from Y to Z is a dinatural transformation from X Y to Z. We define O X Y to be w ob W Set(Sw Xw,Y w) (Thus there is no naturality condition on a computation-morphism, nor can there be, because S is not a functor.) Identity, composition and reindexing are defined in the evident way. 5. CBPV Adjunction Models 5.1. Universal Properties. So far, the only type constructors we can interpret are 1 and. The rest are interpreted using the following universal properties. The notation f g means f;g in the case that g is a value morphism Definition. In a CBPV judgement model (C, D, O), UB a right adjunctive for a D-object B is a C-object V and an O-morphism such that the functions force V B, C(X,V ) O X B for all X (9) f f force are isomorphisms FA a left adjunctive for a C-object A is a D-object V and an O-morphism such that the functions return A V, D X (V,Y ) O X A Y for all X,Y (10) h (π X,A return); (π X,A h) are isomorphisms i I A i a distributive coproduct for a family {A i } i I of C-objects is a C-object V and, for each i I, a C-morphism A i in i V, such that the functions are isomorphisms C(X V,Y ) i I C(X A i,y ) for all X,Y (11) O X V Y i I O X A i Y for all X,Y (12) D X V (Y,Z) i I D X A i (Y,Z) for all X,Y,Z (13) f λi.((x in i ) f)

14 14 Proof. products The isomorphism D X (F1,Y ) = O X 1 Y = O X Y maps h to return;h, by calculation. So the diagram D X (F1,V ) i I D X(F1,B i ) = O X (F1,V ) = i I O XB i commutes, by calculation. So if the top arrow is an isomorphism, the bottom arrow must be too. exponentials Similar, using the diagram D X (F1,V ) = O X V D X A (F1,B) = O X A B distributive coproducts The diagram C X V UY = O X V Y i I C X A i UY = i I O X A i Y commutes by calculation. So if the top arrow is an isomorphism, the bottom arrow must be too. We draw attention to the requirement for (13) to be an isomorphism, which is not eliminated by Prop It was mistakenly omitted in [Lev01] (which merely required C to be countably distributive), and was brought to light by the stack judgement Examples. We now look again at the examples of CBPV judgement models described in Sect. 4.9 to see when they have all the required universal elements. Notice how the strong monad described in [Mog91] is recovered in all cases treated there. For the possible world model, where the strong monad mentioned in [Lev02] is recovered, and for the game model, the strong monad described in [AM98] is recovered.

15 trivial The judgement model given by a cartesian category C is a CBPV adjunction iff C is countably bicartesian closed, i.e. a ccc with all countable coproducts and products. Such trivial models interpret CBPV with no computational effects at all. Indeed, CBPV adjunction can be seen as a generalization of countably bicartesian closed category to accommodate computational effects. cpo This is clearly a CBPV adjunction with FA given as the lift of A, and UB = B. monad/algebra The judgement model given by a strong monad T on a cartesian category C is a CBPV adjunction iff C is countably distributive C has a product for every countable family of T-algebra carriers C has an exponential from every object to every T-algebra carrier. where FA is the free T-algebra on A and UB is the carrier of B. Notice that these conditions imply that C has all Kleisli exponentials and all countable products of Kleisli exponentials. storage, erratic choice, continuations The connectives are given by model i I 1 U F i I storage i I 1 S S i I erratic choice i I 1 P i I continuations i I 1 R R i I possible worlds The 1 connectives are given pointwise. For example, the exponential A B is given at w by Aw Bw, which must be contravariant in w because A is covariant and B is contravariant. For the adjunctives, recall the coslice category w/w, in which an object is a world w together with a morphism w g w. The adjunctives are given at a world w by (F A)w = (U B)w = (w,g) w/w (w,g) w/w (Sw Aw ) (Sw Bw ) 15 and at a morphism x f w by ((FA)f)(w, w g w,s,a) = (w, (f;g),s,a) (((UB)f)α)(w, w g w,s ) = α(w, (f;g),s )

16 5.8. General Storage Models. We can generalize some of the examples in Sect. 5.7 (global store, possible worlds, continuations) into ways of building CBPV models from other CBPV models. This enables us to model combinations of effects, rather like the monad transformers of [CM93]. For global store, suppose that S is a C-object in a CBPV adjunction (C, D, O,...). Then we obtain a storage model (C, D, O ) where O X Y is defined to be O S XY. The 1 connectives are unchanged, and the adjunctives are given by F A = F(S A) U B = U(S B) For possible worlds, suppose we are given a CBPV adjunction (C, D, O,...), a category W with countably many objects and morphisms, and a C-object Sw for each w W. We then obtain a possible world model (C, D, O ) as follows. C is defined to be [W, C] a D -object is a functor from W op to D 1 a D morphism Y h X such that the diagram over Xw Z provides for each world w a morphism Y w hw Xw 16 Zw, ( ) Y f Y w hw Y w (Xf) (hw ) Zw Zw ( ) Zf commutes for each w f w O X Y is defined to be w W O Sw Xw Y w identity, composition and reindexing are defined in the evident way the 1 connectives are given pointwise the adjunctives are given at a world w by (F A)w = F (U B)w = U (w,g) w/w (w,g) w/w and at a morphism x f w in the evident way. (Sw Aw ) (Sw Bw ) Again, the global store construction is the special case that W is the unit category.

17 5.9. General Continuation Models. To see the general construction of continuation models, it is helpful to introduce a calculus. Let us write CBPV+R for CBPV extended with a free computation type identifier R. This has a special fragment called Jump-With-Argument (JWA), shown in Fig (Many similar continuation calculi appear in the literature [Dan92, LRS93, SF93, Sel01, Thi97].) JWA contains two kinds of terms: values and nonreturning commands. We write Γ n M to say that M is a nonreturning command in context Γ. 17 Types A ::= A i I A i 1 A A Judgements Terms: Γ,x : A, Γ v x : A Γ v V : A Γ n M Γ v V : A Γ,x : A n M Γ n let V be x. M Γ v V : Aî Γ v (î,v ) : i I A i î I Γ v V : i I A i Γ,x i : A i n M i i I Γ n pm V as {...,(i,x i ).M i,...} Γ v V : A Γ v V : A Γ v V : A A Γ,x : A,y : A n M Γ v (V,V ) : A A Γ,x : A n M Γ v γx.m : A Γ n pm V as (x,y).m Γ v V : A Γ v W : A Γ n V ր W Figure 4: Syntax of Jump-With-Argument A is U(A R) Γ n M is Γ c M : R γx.m is thunk λx.m V ր W is V force W Figure 5: JWA as fragment of CBPV+R We can substitute a context morphism Γ q into a command n M, giving a

18 18 command Γ n q M. This operation satisfies id M = M p q M = (p;q) M For JWA, as for CBPV and CBV, we require a cartesian category to model the values. For nonreturning commands, we require a functor N : C op Set, also called a left C-module. This can be describe in concrete terms Definition. 1. A left C-module N consists of for each A ob C, a set N A of N-morphisms from A, written A g for each C-morphism A f A and N-morphism A g morphism A f;g, a composite N- satisfying identity and associativity laws where g is a N-morphism. id; g = g (f;f );g = f; (f ;g) 2. A JWA judgement model is a cartesian category C together with a left C-module N. The only connectives we can interpret in a JWA judgement model are 1 and. To interpret the other connectives, we once again need appropriate universal properties Definition. In a JWA judgement model (C, N), A a jumpwith for a C-object A is a C-object V and a N-morphism V A jump such that the functions C X V N X A for all X (18) f (f A) jump are isomorphisms. i I A i a distributive coproduct for a family {A i } i I of C-objects is a C-object V and, for each i I, a C-morphism A i in i V, such that the functions are isomorphisms. C(X V,Y ) i I C(X A i,y ) for all X,Y (19) N X V i I N X A i for all X (20) f λi.((x in i ) f)

19 A JWA model is a JWA judgement model with all jumpwiths and all countable distributive coproducts, and we write and for the operations on objects. There is some redundancy in the definition of JWA model, by analogy with Prop Proposition. In a JWA judgement model with all jumpwiths, distributive coproducts if the functions (19) are all isomorphisms then so are the functions (20). Proof. The isomorphism 19 C X 1 = N X 1 = N X maps f to (f, ( )) jump, by calculation. So the diagram C X V 1 i I C X A i 1 = N X V = i I N X A i commutes by calculation. So if the top arrow is an isomorphism, the bottom must be too. Now we know what a JWA model is, we can define 2 constructions. 1. Given a CBPV model (C, D, O) and D-object R, we obtain a JWA model (C, N) by setting N X = O X R. The 1 connectives are unchanged, and jumpwiths are given by A = U(A R). 2. Suppose we are given a JWA model (C, N). Then we obtain a CBPV adjunction (C, D, O ) where D is (self C) op and O X Y is defined to be N X Y. The 1 connectives are unchanged, and the other connectives are given by F A = A U B = B A B = A B i I B i = i I B i 6. Game Models We wish to show the game semantics of [AHM98] to be a CBPV adjunction. It is convenient to do this first without the bracketing condition, in the manner of [Lai97], which is easier because it is a continuation model. Then we adapt the definitions to incorporate the bracketing condition. It is also possible to treat models constrained by visibility and innocence in the manner of [HO00, AM98], but we do not describe this here.

20 6.1. JWA Model Without Bracketing Condition. We first construct the JWA model. The basic definitions are standard: 6.2. Definition. 1. An (unlabelled) arena is a countable forest, i.e. a countable set R of tokens together with a function enabler : R { } + R such that, for every token r, the sequence r = r 0 enabler r 1 enabler eventually reaches. We usually write R instead of (R, enabler), and we write rt R for the set of tokens enabled by. 2. A play in an arena R is a sequence of tokens t 0,...,t n 1, together with a function justifier : {0,...,n 1} {, 0,...,n 1} such that, for all i {0,...,n 1}, we have justifier i < i enabler t i = t justifier i justifier i is even iff i is odd where is taken to be odd and < 0, and t is defined to be. By contrast with the usual accounts, we make Player go first. So, for a play of length n, we call i {0,...,n 1} a P-move if i is even and an O-move if i is odd. Similarly, we say the play is awaiting P if n is even, and awaiting O if n is odd Definition. A strategy for an arena R is a prefix-closed set σ of O-awaiting plays that is deterministic i.e. if sm σ and sn σ then m = n. We write strat R for the set of strategies on R. This enables us to describe the homsets of the thread-independent category G of [AHM98] (without the bracketing condition) by G(R,S) = strat(r S s ) (21) s rt S where we write for disjoint union of arenas, and S s for the arena of tokens in S strictly below s. We then define our JWA model (C, N) in the manner of [AM98]. Thus, a C-object is a countable family of arenas, and the homsets are given by C({R i } i I, {S j } j J ) = G(R i,s j ) i I j J 20 N {Ri } i I = i I strat R i

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