This tutorial illustrates uses of Z3's fixedpoint engine. The following papers μZ - An Efficient Engine for Fixed-Points with Constraints. (CAV 2011) and Generalized Property Directed Reachability (SAT 2012) describe some of the main features of the engine.

Please send feedback, comments and/or corrections to nbjorner@microsoft.com.

This tutorial covers some of the fixedpoint utilities available with Z3. The main features are a basic Datalog engine, an engine with relational algebra and an engine based on a generalization of the Property Directed Reachability algorithm.

The default fixed-point engine is a bottom-up Datalog engine. It works with finite relations and uses finite table representations as hash tables as the default way to represent finite relations.

```
fp = Fixedpoint()
a, b, c = Bools('a b c')
fp.register_relation(a.decl(), b.decl(), c.decl())
fp.rule(a,b)
fp.rule(b,c)
fp.set(engine='datalog')
print ("current set of rules\n", fp)
print (fp.query(a))
fp.fact(c)
print ("updated set of rules\n", fp)
print (fp.query(a))
print (fp.get_answer())
```

fp = Fixedpoint()creates a context for fixed-point computation.

fp.register_relation(a.decl(), b.decl(), c.decl())Register the relations

fp.rule(a,b)Create the rule that

fp.rule(Thehead,[body1,...,bodyN],name)

fp.query(a)Asks if

fp.fact(c)Add a fact (shorthand for

It is also possible to get an explanation for a derived query. For the finite Datalog engine, an explanation is a trace that provides information of how a fact was derived. The explanation is an expression whose function symbols are Horn rules and facts used in the derivation.

```
fp = Fixedpoint()
a, b, c = Bools('a b c')
fp.register_relation(a.decl(), b.decl(), c.decl())
fp.rule(a,b)
fp.rule(b,c)
fp.fact(c)
fp.set(generate_explanations=True, engine='datalog')
print (fp.query(a))
print (fp.get_answer())
```

Relations can take arguments. We illustrate relations with arguments using edges and paths in a graph.

```
fp = Fixedpoint()
fp.set(engine='datalog')
s = BitVecSort(3)
edge = Function('edge', s, s, BoolSort())
path = Function('path', s, s, BoolSort())
a = Const('a',s)
b = Const('b',s)
c = Const('c',s)
fp.register_relation(path,edge)
fp.declare_var(a,b,c)
fp.rule(path(a,b), edge(a,b))
fp.rule(path(a,c), [edge(a,b),path(b,c)])
v1 = BitVecVal(1,s)
v2 = BitVecVal(2,s)
v3 = BitVecVal(3,s)
v4 = BitVecVal(4,s)
fp.fact(edge(v1,v2))
fp.fact(edge(v1,v3))
fp.fact(edge(v2,v4))
print ("current set of rules", fp)
print (fp.query(path(v1,v4)), "yes we can reach v4 from v1")
print (fp.query(path(v3,v4)), "no we cannot reach v4 from v3")
```

fp.declare_var(a,b,c)to instrument the fixed-point engine that

A more entertaining example of using the basic fixed point engine
is to solve the Rush Hour puzzle. The puzzle is about moving
a red car out of a gridlock. We have encoded a configuration
and compiled a set of rules that encode the legal moves of the cars
given the configuration. Other configurations can be tested by
changing the parameters passed to the constructor for

```
class Car():
def __init__(self, is_vertical, base_pos, length, start, color):
self.is_vertical = is_vertical
self.base = base_pos
self.length = length
self.start = start
self.color = color
def __eq__(self, other):
return self.color == other.color
def __ne__(self, other):
return self.color != other.color
dimension = 6
red_car = Car(False, 2, 2, 3, "red")
cars = [
Car(True, 0, 3, 0, 'yellow'),
Car(False, 3, 3, 0, 'blue'),
Car(False, 5, 2, 0, "brown"),
Car(False, 0, 2, 1, "lgreen"),
Car(True, 1, 2, 1, "light blue"),
Car(True, 2, 2, 1, "pink"),
Car(True, 2, 2, 4, "dark green"),
red_car,
Car(True, 3, 2, 3, "purple"),
Car(False, 5, 2, 3, "light yellow"),
Car(True, 4, 2, 0, "orange"),
Car(False, 4, 2, 4, "black"),
Car(True, 5, 3, 1, "light purple")
]
num_cars = len(cars)
B = BoolSort()
bv3 = BitVecSort(3)
state = Function('state', [ bv3 for c in cars] + [B])
def num(i):
return BitVecVal(i,bv3)
def bound(i):
return Const(cars[i].color, bv3)
fp = Fixedpoint()
fp.set(generate_explanations=True)
fp.declare_var([bound(i) for i in range(num_cars)])
fp.register_relation(state)
def mk_state(car, value):
return state([ (num(value) if (cars[i] == car) else bound(i))
for i in range(num_cars)])
def mk_transition(row, col, i0, j, car0):
body = [mk_state(car0, i0)]
for index in range(num_cars):
car = cars[index]
if car0 != car:
if car.is_vertical and car.base == col:
for i in range(dimension):
if i <= row and row < i + car.length and i + car.length <= dimension:
body += [bound(index) != num(i)]
if car.base == row and not car.is_vertical:
for i in range(dimension):
if i <= col and col < i + car.length and i + car.length <= dimension:
body += [bound(index) != num(i)]
s = "%s %d->%d" % (car0.color, i0, j)
fp.rule(mk_state(car0, j), body, s)
def move_down(i, car):
free_row = i + car.length
if free_row < dimension:
mk_transition(free_row, car.base, i, i + 1, car)
def move_up(i, car):
free_row = i - 1
if 0 <= free_row and i + car.length <= dimension:
mk_transition(free_row, car.base, i, i - 1, car)
def move_left(i, car):
free_col = i - 1;
if 0 <= free_col and i + car.length <= dimension:
mk_transition(car.base, free_col, i, i - 1, car)
def move_right(i, car):
free_col = car.length + i
if free_col < dimension:
mk_transition(car.base, free_col, i, i + 1, car)
# Initial state:
fp.fact(state([num(cars[i].start) for i in range(num_cars)]))
# Transitions:
for car in cars:
for i in range(dimension):
if car.is_vertical:
move_down(i, car)
move_up(i, car)
else:
move_left(i, car)
move_right(i, car)
def get_instructions(ans):
lastAnd = ans.arg(0).children()[-1]
trace = lastAnd.children()[1]
while trace.num_args() > 0:
print (trace.decl())
trace = trace.children()[-1]
print (fp)
goal = state([ (num(4) if cars[i] == red_car else bound(i))
for i in range(num_cars)])
fp.query(goal)
get_instructions(fp.get_answer())
```

Below we give a simple example that illustrates a loop at location `l0`.
The loop is incremented as long as the loop counter does not exceed an upper
bound. Using the combination of bound and interval domains
we can collect derived invariants from the loop and we can also establish
that the state after the loop does not exceed the bound.

```
I = IntSort()
B = BoolSort()
l0 = Function('l0',I,I,B)
l1 = Function('l1',I,I,B)
s = Fixedpoint()
s.set(engine='datalog',compile_with_widening=True,
unbound_compressor=False)
s.register_relation(l0,l1)
s.set_predicate_representation(l0, 'interval_relation', 'bound_relation')
s.set_predicate_representation(l1, 'interval_relation', 'bound_relation')
m, x, y = Ints('m x y')
s.declare_var(m, x, y)
s.rule(l0(0,m), 0 < m)
s.rule(l0(x+1,m), [l0(x,m), x < m])
s.rule(l1(x,m), [l0(x,m), m <= x])
print ("At l0 we learn that x, y are non-negative:")
print (s.query(l0(x,y)))
print (s.get_answer())
print ("At l1 we learn that x <= y and both x and y are bigger than 0:")
print (s.query(l1(x,y)))
print (s.get_answer())
print ("The state where x < y is not reachable")
print (s.query(And(l1(x,y), x < y)))
```

set_option(dl_compile_with_widening=True)to instrument Z3 to apply abstract interpretation

set_option(dl_engine=1)The version in Z3 applies to Horn clauses with arithmetic and Boolean domains. When using arithmetic you should enable the main abstraction engine used in Z3 for arithmetic in PDR.

set_option(dl_pdr_use_farkas=True)The engine also works with domains using algebraic data-types and bit-vectors, although it is currently not overly tuned for either. The PDR engine is targeted at applications from symbolic model checking of software. The systems may be infinite state. The following examples also serve a purpose of showing how software model checking problems (of safety properties) can be embedded into Horn clauses and solved using PDR.

McCarthy's 91 function illustrates a procedure that calls itself recursively twice. The Horn clauses below encode the recursive function:

mc(x) = if x > 100 then x - 10 else mc(mc(x+11))The general scheme for encoding recursive procedures is by creating a predicate for each procedure and adding an additional output variable to the predicate. Nested calls to procedures within a body can be encoded as a conjunction of relations.

```
mc = Function('mc', IntSort(), IntSort(), BoolSort())
n, m, p = Ints('n m p')
fp = Fixedpoint()
fp.declare_var(n,m)
fp.register_relation(mc)
fp.rule(mc(m, m-10), m > 100)
fp.rule(mc(m, n), [m <= 100, mc(m+11,p),mc(p,n)])
print (fp.query(And(mc(m,n),n < 90)))
print (fp.get_answer())
print (fp.query(And(mc(m,n),n < 91)))
print (fp.get_answer())
print (fp.query(And(mc(m,n),n < 92)))
print (fp.get_answer())
```

We can also prove invariants of reactive systems. It is convenient to encode reactive systems as guarded transition systems. It is perhaps for some not as convenient to directly encode guarded transitions as recursive Horn clauses. But it is fairly easy to write a translator from guarded transition systems to recursive Horn clauses. We illustrate a translator and Lamport's two process Bakery algorithm in the next example.

```
set_option(relevancy=0,verbose=1)
def flatten(l):
return [s for t in l for s in t]
class TransitionSystem():
def __init__(self, initial, transitions, vars1):
self.fp = Fixedpoint()
self.initial = initial
self.transitions = transitions
self.vars1 = vars1
def declare_rels(self):
B = BoolSort()
var_sorts = [ v.sort() for v in self.vars1 ]
state_sorts = var_sorts
self.state_vals = [ v for v in self.vars1 ]
self.state_sorts = state_sorts
self.var_sorts = var_sorts
self.state = Function('state', state_sorts + [ B ])
self.step = Function('step', state_sorts + state_sorts + [ B ])
self.fp.register_relation(self.state)
self.fp.register_relation(self.step)
# Set of reachable states are transitive closure of step.
def state0(self):
idx = range(len(self.state_sorts))
return self.state([Var(i,self.state_sorts[i]) for i in idx])
def state1(self):
n = len(self.state_sorts)
return self.state([Var(i+n, self.state_sorts[i]) for i in range(n)])
def rho(self):
n = len(self.state_sorts)
args1 = [ Var(i,self.state_sorts[i]) for i in range(n) ]
args2 = [ Var(i+n,self.state_sorts[i]) for i in range(n) ]
args = args1 + args2
return self.step(args)
def declare_reachability(self):
self.fp.rule(self.state1(), [self.state0(), self.rho()])
# Define transition relation
def abstract(self, e):
n = len(self.state_sorts)
sub = [(self.state_vals[i], Var(i,self.state_sorts[i])) for i in range(n)]
return substitute(e, sub)
def declare_transition(self, tr):
len_s = len(self.state_sorts)
effect = tr["effect"]
vars1 = [Var(i,self.state_sorts[i]) for i in range(len_s)] + effect
rho1 = self.abstract(self.step(vars1))
guard = self.abstract(tr["guard"])
self.fp.rule(rho1, guard)
def declare_transitions(self):
for t in self.transitions:
self.declare_transition(t)
def declare_initial(self):
self.fp.rule(self.state0(),[self.abstract(self.initial)])
def query(self, query):
self.declare_rels()
self.declare_initial()
self.declare_reachability()
self.declare_transitions()
query = And(self.state0(), self.abstract(query))
print (self.fp)
print (query)
print (self.fp.query(query))
print (self.fp.get_answer())
# print self.fp.statistics()
L = Datatype('L')
L.declare('L0')
L.declare('L1')
L.declare('L2')
L = L.create()
L0 = L.L0
L1 = L.L1
L2 = L.L2
y0 = Int('y0')
y1 = Int('y1')
l = Const('l', L)
m = Const('m', L)
t1 = { "guard" : l == L0,
"effect" : [ L1, y1 + 1, m, y1 ] }
t2 = { "guard" : And(l == L1, Or([y0 <= y1, y1 == 0])),
"effect" : [ L2, y0, m, y1 ] }
t3 = { "guard" : l == L2,
"effect" : [ L0, IntVal(0), m, y1 ]}
s1 = { "guard" : m == L0,
"effect" : [ l, y0, L1, y0 + 1 ] }
s2 = { "guard" : And(m == L1, Or([y1 <= y0, y0 == 0])),
"effect" : [ l, y0, L2, y1 ] }
s3 = { "guard" : m == L2,
"effect" : [ l, y0, L0, IntVal(0) ]}
ptr = TransitionSystem( And(l == L0, y0 == 0, m == L0, y1 == 0),
[t1, t2, t3, s1, s2, s3],
[l, y0, m, y1])
ptr.query(And([l == L2, m == L2 ]))
```

```
# let max max2 x y z = max2 (max2 x y) z
# let f x y = if x > y then x else y
# assert (f (max f x y z) x) = (max f x y z)
Expr = Datatype('Expr')
Expr.declare('Max')
Expr.declare('f')
Expr.declare('I', ('i', IntSort()))
Expr.declare('App', ('fn',Expr),('arg',Expr))
Expr = Expr.create()
Max = Expr.Max
I = Expr.I
App = Expr.App
f = Expr.f
Eval = Function('Eval',Expr,Expr,Expr,BoolSort())
x = Const('x',Expr)
y = Const('y',Expr)
z = Const('z',Expr)
r1 = Const('r1',Expr)
r2 = Const('r2',Expr)
max = Const('max',Expr)
xi = Const('xi',IntSort())
yi = Const('yi',IntSort())
fp = Fixedpoint()
fp.register_relation(Eval)
fp.declare_var(x,y,z,r1,r2,max,xi,yi)
# Max max x y z = max (max x y) z
fp.rule(Eval(App(App(App(Max,max),x),y), z, r2),
[Eval(App(max,x),y,r1),
Eval(App(max,r1),z,r2)])
# f x y = x if x >= y
# f x y = y if x < y
fp.rule(Eval(App(f,I(xi)),I(yi),I(xi)),xi >= yi)
fp.rule(Eval(App(f,I(xi)),I(yi),I(yi)),xi < yi)
print (fp.query(And(Eval(App(App(App(Max,f),x),y),z,r1),)
Eval(App(f,x),r1,r2),
r1 != r2))
print (fp.get_answer())
```