Rel
REL CONCEPTS
Recursion

# Recursion

This concept guide demonstrates how to write and successfully use recursive relations.

## Goal#

The goal of this concept guide is to demonstrate how to write and successfully use recursive relations (which means relations that depend on themselves) in Rel.

## Preliminaries#

This concept guide uses the following Rel features:

It is good to know how ICs work, but not required for understanding this guide. The Advanced Examples section uses modules.

## Introduction#

A program or a query is called recursive when a predicate is defined in terms of itself. For many computational problems, recursion helps to reduce complexity and to express solutions in a cleaner and more understandable way. A common side effect is that recursive solutions are often shorter, more compact, and easier to maintain.

Rel supports recursive computations, which is also a key ingredient for making the language Turing complete. This means Rel can be used to perform any computation that any other programming language supports. This does not mean that expressions in Rel are more compact or easier to write even though we believe that is generally the case.

With the capability to express queries recursively, Rel is able to perform a variety of computations that at first glance one would not expect from a database language:

• Iterative algorithms can be expressed as recursive Rel programs (e.g.: Fibonacci, Prim’s Minimal spanning tree);

• Relations requiring a variable recursion depth (aka an unbounded query length) can be expressed more compactly and efficiently (e.g., transitive closure, PageRank, moving averages).

To define recursive computations, we need the following ingredients:

1. A base case, and
2. A recursive rule.

The base case (or cases) is the starting point of the recursive computation. The recursive rule (or rules) is the heart of the recursive computation, which references itself.

### Termination of Recursive Computations#

Computationally, we need a third ingredient in order to have a successful recursive algorithm: termination conditions, which ensure that the recursive computation doesn’t run forever.

Generally, a recursive computation in Rel terminates when a fixpoint is reached, meaning that the elements in a relation don’t change anymore from one iteration to the next (see Section Behind the Scenes for more details).

More formally, this means the solution to the recursive computation is a relation (or a set of relations) R, which is a fixpoint of the following definition:

def R(x...) = base_case(x...) or recursive_rule[R](x...)

A recursive computation defining an infinite relation (or infinite function), such as the Fibonacci sequence, currently requires a domain restriction for the computation to terminate (see examples below).

For certain recursive computations no explicit domain restriction is needed because the definitions naturally reach a fixpoint. Graph reachability (see below) is such a problem because the output domain, which is all possible pairs of graph nodes, is finite and the recursion is monotonically increasing.

(We say that the recursion is monotonically increasing if at each iteration we can only add new elements, and never remove any.)

Future releases of Rel will relax this restriction, and compute the desired results in an on-demand fashion, without having to compute the entire (infinite) relation.

## Introductory Examples#

### Recursive Computation on Graphs#

One of the most fundamental graph queries asks if there is a path between two nodes. Such questions appear frequently in the real world. In terms of international travel, this question could be

Can I fly from Providence to Honolulu?

Let’s try to answer that question in Rel. First we need a few airports:

// install

def airport = {
"Providence (PVD)"; "New York (JFK)"; "Boston (BOS)";
"San Francisco (SFO)"; "Los Angeles (LAX)"; "Honolulu (HNL)";
}

and a flight network,

// install

def directional_flight = {
("Providence (PVD)", "Boston (BOS)"); ("Boston (BOS)", "San Francisco (SFO)");
("San Francisco (SFO)", "Honolulu (HNL)"); ("New York (JFK)", "Los Angeles (LAX)");
}

def flight(a, b) = directional_flight(a, b) or directional_flight(b, a)

// integrity constraint
ic flight_between_airports(a, b) = flight(a, b) implies airport(a) and airport(b)

where we introduced directional flights, directional_flight, and bidirectional flights, flight. The integrity constraint checks that flights can only occur between airports.

Now, we can calculate all pairs of airports that are (directly or indirectly) connected with each other,

// install

def connected(a, b) = flight(a, b) // the base case
def connected(a, b) = exists(c : connected(a, c) and flight(c, b))

which is the recursive part of the problem.

// query

def output =
if connected("Providence (PVD)", "Honolulu (HNL)") then
"yes"
else
"no"
end

// This integrity constraint will fail if we don't compute that
// PVD and HNL are connected:
ic {equal(output, "yes")}

Let’s ask another question and check if we can fly from Boston (BOS) to Los Angeles (LAX).

// query

def output =
if connected("Boston (BOS)", "Los Angeles (LAX)") then
"yes"
else
"no"
end

// This integrity constraint will fail if we compute that
// BOS and LAX are connected:
ic {equal(output, "no")}

The answer is no, because in our toy world LAX and JFK are connected with each other but isolated from all the other airports.

For the mathematical enthusiasts, the problem of computing all pairs of connected nodes in a graph is called the transitive closure problem. In our example, the edges of the graph are defined in the flight relation.

### Single Recursion#

One of the simplest mathematical recursion formulas is the definition of the factorial $n! = n \cdot (n-1) \cdot \ldots \cdot 1$ for $n\in \mathbb{N}_+$, which fulfills the recursive rule, $n! = n \cdot (n-1)!$. In Rel this reads

// query

def F = 1 // the base case
def F[n] = n * F[n - 1], n <= 10

def output = F

Relation F has arity 2, where the first entry is the iteration step n and the second entry is the value of n!.

The definition contains also all ingredients for a successful recursion:

• Base case: def F = 1 is the starting point and translates to 1! = 1.
• Recursive rule: def F[n] = n * F[n - 1] translates to $n! = n\cdot(n-1)!$.
• Termination condition: n<=10 makes the output domain finite, forces the fixpoint to be reached when $n$ hits 10, and consequently terminates the recursive evaluation. (In the future, once demand transformations are implemented, this explicit statement of the termination condition will not be required anymore.)

### Multiple Recursion#

#### Fibonacci Sequence#

Multiple recursion, where the recursive rule contains multiple self-references, can also be easily expressed in Rel.

The most famous example is the Fibonacci sequence $F(n) = F(n-1) + F(n-2)$ with $F(0)=0$ and $F(1)=1$. In Rel this relation reads:

// query

def F = 0  // base cases
def F = 1  // base cases
def F[n] = F[n-1] + F[n-2], n<=10

def output = F

We can easily see, all ingredients for a successful recursive computation are present.

#### Connected Nodes in a Graph#

In our previous international travel example, we could have defined the connected relation using multiple recursions:

// query

def connected(a, b) = flight(a, b) // the base case
def connected(a, b) = connected[a].connected(b) // recursive rule

Here the recursive rule contains also two self-references and reads: Two airports a, b are connected if there exists a third airport that is connected to both airports.

### Negation in Recursion#

Recursive dependencies can be made as complex as needed, and can go well beyond just asking for a single element of a previous iteration (e.g.: F[n - 1]).

The Recamán sequence is a nice example to demonstrate a more complex recursive computation. The sequence is defined as:

$a_n = \left\{\begin{array}{ll} 0 & \text{if}\ n=0 \\ a_{n-1}-n & \text{if}\ a_{n-1}-n>0\ \text{and the value isn't already in the sequence} \\ a_{n-1}+n & \text{otherwise} \\ \end{array}\right.$

The recursive case requires that we check that the potential next value isn’t already in the sequence, and if it is then assign a different value.

The Recamán sequence can be visualized in a very neat way by connecting subsequent values with an arch. One way to define the Recamán sequence In Rel is as follows.

// query

@inline
def reject(n,x) = x < 0 or exists(m: recaman(m, x) and m < n)

def recaman = 0
def recaman[n](x) = recaman[n - 1] - n = x and not reject(n, x) and 0 < n < 20
def recaman[n](x) = recaman[n - 1] + n = x and reject(n, x - 2 * n) and 0 < n < 20

def output = recaman
// This IC verifies the correctness of the results by comparing with hand-computed results.
ic recaman_sequence = equal(
recaman[_],
{0;1;3;6;2;7;13;20;12;21;11;22;10;23;9;24;8;25;43;62;}
)

where we limited ourselves to calculating only the first 20 terms. We used several logical elements here, i.e.:

• negation,
• existential quantification,
• disjunction,

to express the recursive Recamán rule.

Additionally, we used the @inline functionality to factor out the condition evaluation in a separate relation (reject) helping to keep the main recursive rule compact and readable.

Finally, we added an integrity constraint (IC) to check that the set of computed values is correct. (Note that this IC does not check that the values are in the correct order, but still gives us additional confidence in the results.)

Rel supports complex recursive rules that may involve multiple relations (aka mutual recursion) that depend on each other and may be recursive themselves.

Before diving into them, let’s look at relations that are not monotonically increasing in cardinality as the iteration advances. This is possible because elements can be not just added but also deleted during an iterative step.

### Recursive Rules that Eliminate Elements#

For all the examples seen so far, the number of elements in the recursive relation (also known as cardinality) is larger at the end of the computation. However, Rel can also delete, not just add, elements during an iteration, so it is also possible to finish with fewer elements in the relation than at the start.

The following Rel program has two recursive rules for D. The first one adds the elements one through 10, if D is empty. The second one keeps only elements that are smaller than or equal to three, or different from the maximum element in D. The effect is that the largest element is iteratively removed until only the elements 1; 2; 3 remain:

// query

def D(n) = range[1, 10, 1](n) and empty(D)
def D(n) = D(n) and (n <= 3 or n != max[D])

def output = D


Notice that the program contains only the two recursive rules and does not explicitly state a base case. This means that the base case is, by default, an empty relation D. The first rule adds the numbers one through 10 if and only if D is empty.

The key difference between the two recursive rules is: The first rule probes the non-existence of elements whereas the second rule depends on an aggregated view of existing elements. Either of these aspects (negation or aggregation) is enough to achieve a non-monotonic recursion. In the absence of both features, recursion is always monotonic.

### Asymptotic Convergence: PageRank#

Another class of recursive formulas that we can easily implement in Rel is infinite series that converge asymptotically (in the infinite limit, $n→∞$). That is in contrast to the example in the section Recursive Computation on Graphs, where we reached convergence after a finite number of steps.

In practice, we define a convergence criterion, where we stop the iteration if the value(s) between consecutive iterations are closer than a (small) user-defined $ϵ$.

$\| a_{n+1} - a_n \|^2 < ϵ \quad \text{for}\ n>N_\text{converged} \enspace,$

where $N_\text{converged}$ is the number of iterations needed to achieve our desired accuracy.

Many machine learning algorithms fall in this category, where we declare the model as trained once the model parameters and/or the accuracy of the model change only marginally between iterations.

Let’s take the PageRank algorithm with damping as an example. It is defined as

$PR_a = \frac{1-d}{N} + d \sum_{b \in \mathcal{S}_a} \frac{PR_b}{L_b} \enspace ,$

$PR_a$ is the page rank of website $a$, $L_b$ is the number of outbound links from site $b$, $\mathcal{S}_a$ is the collection of websites that have a link to website $a$, $N$ is the total number of websites, and $d$ is a damping parameters, which is usually set around 0.85.

First, define an undirected weighted graph, inspired by Wikipedia.

// install

module graphA
def edge = {
("N1", "B"); ("N1", "E");
("N2", "B"); ("N2", "E");
("N3", "B"); ("N3", "E");
("N4", "E"); ("N5", "E");
("E", "F"); ("F", "E");
("E", "B"); ("E", "D");
("D", "A"); ("D", "B");
("B", "C"); ("C", "B");
("F", "B"); }

def node = x : edge(x, _) or edge(_, x)
end


Here is a visualization of the graph, where the size of a node is proportional to its PageRank.

The PageRank algorithm assumes that sink nodes, which have no outgoing edges, are connected to all other nodes in the graph. We can derive a new graph from the original as follows:

// install

module graphB
def node = graphA:node
def edge = graphA:edge
def sink(n in node) = empty(graphA:edge[n])
def edge(a in node, b in node) = sink(a) // add new edges
end

In Rel, the PageRank algorithm may read:

// install

// parameters
def eps = 10.0^(-10)
def damping = 0.85
def node = graphB:node
def edge = graphB:edge

def node_count = count[node]
def outdegree[x] = count[edge[x]]

def MAX_ITER = 100

// a version of sum that gives 0.0 for the empty set,
// needed if there are edges with no incoming nodes:
@inline
def sum_default[R] = sum[R] <++ 0.0

// pagerank(iteration, site, rank)
def pagerank[0, a in node] = 1.0 / node_count  //base case: equal rank
def pagerank[n, a in node] = // recursive rule
(1-damping)/node_count + damping * sum_default[
pagerank[n-1, b]/outdegree[b] for b where edge(b, a)
],
pagerank(n-1, a, _) // grounding n and a
and not converged(n-1) // termination condition
and n<=MAX_ITER  // safeguard

// track convergence:
def converged(n) =
forall(a in node : (pagerank[n, a] - pagerank[n-1, a])^2 < eps)
and range(1, MAX_ITER, 1, n)

The relation converged stores the iteration numbers for which the convergence criterion is fulfilled. The not converged(n-1) condition in the main recursive definition for pagerank ensures that the iteration stops once convergence is reached. We also added a safeguard condition that stops the recursive computation if 100 iterations have been reached.

Notice that relations pagerank and converged depend on each other and must be solved together.

We use the sum_default relation to ensure that the sum over an empty set returns 0.0 instead of false, which is the default behavior of sum[]. (This is needed if there are no sink nodes in the original graph, and a node with no incoming edges.)

Finally, let’s look at the results. After 62 iterations we have reached our desired accuracy level with the following results:

// query

def max_iteration = max[n : pagerank(n, _, _)]
def did_converge = "yes", converged = max_iteration

def ranks = pagerank[max_iteration]
def sumranks = sum[pagerank[max_iteration]]


max_iteration

did_converge

ranks

sumranks

Since PageRank corresponds to a probability distribution over nodes, the ranks should sum to (approximately) 1.0.

### Prim’s Minimum Spanning Tree#

Prim’s algorithm is a greedy algorithm to calculate a graph’s minimum spanning tree (MST):

1. Initialize a tree with a single node, chosen arbitrarily from the graph.
2. Grow the tree by one edge: of the edges that connect the tree to nodes not yet in the tree, find a minimum-weight edge, and add it to the tree.
3. Repeat step 2, until all nodes are in the tree.

The input to the algorithm is an edge-weighted undirected graph, which is assumed to be connected (that is, it has no disjoint components). This one is inspired by www.geeksforgeeks.org. // install

def graph:edge_weight = {
(0, 1, 4); (1, 2, 9); (2, 3, 7); (3, 4, 9);
(4, 5, 10);(5, 6, 2); (6, 7, 1); (7, 8, 7);
(7, 0, 8); (1, 7, 11); (5, 2, 4);(5, 3, 14);
(2, 8, 2); (6, 8, 6);
}

def graph:node(x) = graph:edge_weight(x,_,_) or graph:edge_weight(_,x,_)

// make graph undirected by adding reverse edges
// (makes the code a little simpler):
def graph:edge_weight(x, y, w) = graph:edge_weight(y, x, w)

The relationgraph:edge_weight contains the edge information. If graph:edge_weight(x,y,w) holds, there is an edge of weight w from x to y. The relation graph:node is the set of nodes.

It is possible to implement Prim’s algorithm with one recursive relation. To demonstrate the interplay of several recursive relations, the code below uses three: Two of them track the edges and nodes in the MST, and one tracks the nodes not yet in the MST:

// query

def mst:node = 0 // initialize MST with an arbitrary node

// collect all nodes that have been already reached by the MST
def mst:node(x) = mst:edge(x, _) or mst:edge(_, x)

// main recursive rule
def mst:edge(x, y) = just_one[
mst:node(x) and
unvisited(y) and
graph:edge_weight[x, y] = lightest_edge_weight[unvisited, mst:node]
]

@inline
def just_one[R] = (top[1,R])

// don't remove edges that have been added to the MST:
def mst:edge = mst:edge

// list of nodes not in the MST yet:
def unvisited(x) = graph:node(x) and not mst:node(x)

// smallest weight of an edge between visited and unvisited nodes:
@inline
def lightest_edge_weight[S, T] = min[v : v=graph:edge_weight[S, T]]

// check that all nodes are visited:
ic { empty(unvisited) }
// the number of edges in a spanning tree is the number of nodes minus one:
ic { count[mst:edge] = count[graph:node] - 1 }

def output:edge = mst:edge
def output:weight = sum[mst:edge <: graph:edge_weight]


The main recursive part of this algorithm involves the three relations mst:node, mst:edge, and unvisited. At each iteration, mst:edge includes all the edges already added (mst:edge(x, y) = mst:edge(x, y)), plus one new edge, which is a lightest edge connecting a visited node, mst:node(x), to an unvisited node, unvisited(y).

The just_one higher-order inline relation makes sure only one edge is added at a time. In general, there could be two such edges, with the same minimum weight, leading to the same unvisited node; adding both edges would create a cycle, which must be avoided for the result to be a tree.

The recursive dependencies are quite non-trivial here because the three relations refer to each other in a cyclical way: unvisited depends on mst:node, which depends on mst:edge, which in turn depends on mst:node and unvisited again. The only direct recursive dependency is def mst:edge = mst:edge. This rule ensures that edges, once inserted, are not removed in a future iteration.

This safeguard is needed because the edge tuple (x,y) in the other mst:edge rule requires that y is unvisited (not in mst:node). With only that rule, new edges would be removed from mst[:edge] in future iterations, when y is in visited.

Now, the condition that y should not be in mst:node might be fulfilled again, entering a cycle that never ends. (See section Common Pitfalls for more details on never-ending recursive computations). This safeguard prevents all that.

unvisited is a nice example of a relation that decreases in size as the recursive evaluation proceeds (see also the section Recursive Rules that Eliminate Elements. Initially, unvisited contains all the nodes in the graph, but once the recursive evaluation is done, unvisited is actually empty, which we also double check with an integrity constraint, ic {not unvisited(_)}.

The edge weights of the MST are not stored in mst because we can look them up in graph[:edge_weight]. Let’s visualize the minimal spanning tree we just calculated. ### Logging Recursive Progress#

Normally, we can only access the relation after the recursive evaluation has finished (aka converged). It would be great to be able to see also the intermediate progress to better understand and debug the logic we write.

Take the example above where elements are successively removed until only three elements are left. See the Recursive Rules That Eliminate Elements section. If you remove the the condition, n<=3, you create a recursive algorithm that never converges. See the Common Pitfalls section.

Let’s see if this is actually what is happening by creating a logging relation Log that records the size of D after each iteration. To avoid the endless loop we add a condition that forces the recursive evaluation to terminate after 10 iterations have been reached.

// install

// number of iterations
def Niter = 10

// endless loop
def D(n) = n=range[1, 3, 1] and not D(_) and continue
def D(n) = D(n) and n!=max[D] and continue

// logging
def Log = 0,0 : not Log(_, _) // dummy initialization
def Log(n, d_size) =
n = max[first[Log]]+1 and
d_size = sum[count[D]; 0] and
continue

def Log(x, y) = Log(x, y) and x > 0 // don't remove entries

// termination condition
def continue = count[Log] < Niter

Note, we also modified the relation a bit and we insert 1;2;3 instead of 1;2;...;10 from the original version above. Let’s have look if Log really contains the size/cardinality of relation D after each iteration:

// query

Log

This confirms that D starts with 3 elements and decreases to 0 (aka empty), at which point the first recursive rule holds again and D is again initialized with 1;2;3. This cycle repeats forever, except that in our case we terminate the iteration when the relation continue turns false, which is the case when our logging relation Log contains at least Niter=10 entries.

## Common Pitfalls#

Probably the single most common problem users will face when using recursion is non-convergent computation.

There are two main types of non-convergence:

• Unbounded domain: Recursive computations where there is no fixpoint or a fixpoint that takes infinitely many iterations to reach. PageRank is an example where the fixpoint lies at infinity. The Fibonacci Sequence and calculating the Length of All Paths in a Cyclical Graph are examples of computation with no fixpoint.

This requires restricting the domain to make sure that the recursive computation terminates after a finite number of iterations.

(This will be addressed in the future with the demand transformation feature — see section Running to Infinity.

• Oscillations: No fixpoints exist even though the output domain is finite. This can occur if the recursive computation gets stuck in a cycle. This situation is much harder to spot and can be quite subtle as the discussion in the MST example above demonstrated. In the future, the query evaluator will be able to detect this behavior and report a failure after a while, when we repeat a solution we have seen before. (See section Stuck in a Cycle).

### Running to Infinity#

#### Factorial#

How can one end up with an infinite loop? One obvious way is, of course, when one forgets to restrict the output domain in a recursive rule that explores a larger and larger domain space as the recursive computation progresses. An example is the factorial, $n!$. Imagine that we write the factorial but don’t state at which $n$ to stop:

// query

def F = 1
def F[n] = n * F[n-1]

Since Rel computes fixpoints in a bottom-up fashion, deriving new facts from existing ones, the iteration will go on forever, as $n$ grows larger and larger. In the future, this issue will be solved with on-demand evaluation, which only performs computations needed when a specific set of values are requested.

#### Length of All Paths in a Cyclical Graph#

Also in graph problems, we can run in to infinite recursion even though the graph itself is finite. For cyclical graphs, for instance, calculating the lengths of all paths will not terminate and will run forever because the cyclical nature of the graph ensures that one can always find a path of arbitrary length by passing through the loop (that exists in the graph) multiple times.

To demonstrate that point, you can construct a minimal graph with three nodes and one loop between nodes 1 and 2.

// install

def cyclical_graph:node = {1; 2; 3}
def cyclical_graph:edge = {(1, 2); (2, 1); (2, 3)}

// path_length(start_node, end_node, distance)
def path_length = cyclical_graph[:edge], 1  // base case
def path_length(a, c, len) =
path_length(a, b, len1) and
path_length(b, c, len2) and
len = len1 + len2  and
len <= 4  // termination to avoid infinite iteration
from b, len1, len2

To avoid an endless loop, you can insert the condition len <= 4. Inspecting the relation path_length shows that between each node pair you have multiple path lengths increasing in steps of two, which is the length of the loop in the cyclical graph:

// query

path_length

### Stuck in a Cycle#

There exists another way that one can get stuck in an infinite recursive computation. Elements can be added and removed during the iteration. Hence, it is possible to end up in a cycle that never ends.

Let’s take the example in section Recursive Rules that Eliminate Elements where we start with the elements 1 to 10 and remove iteratively the largest one but only if the largest element is larger than 3. If we take this condition away,

// query

def D = n : n=range[1, 10, 1] and not D(_)
def D(n) = D(n) and n != max[D]

we will end up in a endless cycle. Why? Because once we removed the last element from D, we are again fulfilling the initial condition again used to populate the relation in the first place. Now, we are back at the beginning and the cycle repeats itself forever. Such a recursive relation has no fixpoint and will never terminate.

As mentioned above, this problem will be solved in the future and Rel will be able to recognize that a solution has been already visited but the recursive computation has not converged yet.

## Behind the Scenes: Recursion and Fixpoints#

In this section, we discuss in more detail how recursive rules are evaluated in Rel. The content discussed here is not needed to write successful recursive relations. It might be, however, very helpful for writing complex recursive algorithms and understanding why Rel returns what it does.

At a high level, Rel solves the recursive relations $R$ by starting at step 0 with the base case(s), $R_0$, which may be the empty set (in case no explicit base case is defined). The relation $R_0$ represents the starting point of our recursive computation. If $f$ stands for a function mapping relation $S$ to relation $T$, then we can express the converged relation $R$ in the following way:

$R = f(R_{N-1}) = (f \circ f)(R_{N-2}) = \underbrace{(f \circ \ldots \circ f)}_{N}(R_{0})$,

where after $N$ iterative applications of $f$ we have converged to a final result by reaching the fixpoint, $f(R) = R$, and the iteration stops as a repetitive application of $f$ doesn’t change the content of $R$.

Rel solves the recursive computations in a bottom-up fashion where we start with the base case(s) and then apply iteratively the recursive rule until we reach a fixpoint. Hence, it is currently difficult to implement a top-down algorithm. Many (naive) implementations of Divide-and-conquer algorithms (e.g.: mergesort, FFT, and Tower of Hanoi) are solved in a top-down fashion.

Besides the common pitfalls discussed above, we also need to be aware of scenarios where we might have multiple fixpoints.

### Multiple Fixpoints#

A function $f$ can have multiple fixpoints, that is, more than one value of $x$ where $f(x)=x \enspace.$ This is easy to see with mathematical functions, such as polynomials. Take, for instance, the function $f(x) = -2(x-1)^2+2 \enspace.$ It has the fixpoints $x=0$ and $x=\frac{3}{2}$, which are the two points where the function intersects the $y=x$ line. Whether iterative methods can find these values may depend on the initial conditions chosen.

This applies to relations in general, and we can come across it in Rel, particularly when using negation, as we now describe.

#### Negations and Cycles#

Consider the following example, featuring recursion and negation:

// query

def man = "Dilbert"
def single(x) = man(x) and not husband(x)
def husband(x) = man(x) and not single(x)
def output:single = single
def output:husband = husband

We have two different least fixpoints:

• Dilbert is single and not a husband: single(Dilbert) and not husband(Dilbert)
• Dilbert is a husband and not single: husband(Dilbert) and not single(Dilbert)

They are incomparable: neither is smaller than the other. Rel computes one of them, but it can be argued that they are both equally valid.

Note that single depends on husband, which in turn depends on single again, so we have a dependency cycle, with intervening negations.

Models with a cyclical definition that involves negation are called non-stratifiable, and should be avoided when possible. In this case, they lead to multiple least fixpoints. In others, the cycles can lead to non-termination, where Rel will throw an exception.

In general,negations and aggregations should be used with caution in recursive definitions, since they can result in non-monotonic recursion and unintended behaviors.

### Symmetry-Induced Fixpoints#

Symmetry is a fundamental concept in mathematics and physics, and is relevant to properties like the law of conservation of energy. Symmetries in the problem definition are a common cause for multiple fixpoints. The multiple fixpoints in the Dilbert example above come from the symmetry between the definitions of husband and single.

In more complex situations the underlying symmetry might be not that easy to spot. For example:

// query

def circle = 1, 0
def circle = -y, x from x, y where circle(x, y)

We will reach the fixpoint after four iterations. Why? Because we will be back at $(1,0)$ after visiting the points $(0,1), (-1,0)$, and $(0,-1)$.

The reason for this cyclical behavior is that we have encoded a rotational symmetry in our problem. With each iteration we perform a $90^\circ$ rotation starting at $(1,0)$.

Hence, once we visited all the points of the rotational group $C_4$ (modulo the starting point) we will have reached the fixpoint and no new elements will be added to circle.

This toy example shows how the underlying symmetry in our algorithm enforces that we stay within a restricted domain and all elements in our relation are elements of that domain. Only symmetry-breaking operations will be able to escape this domain.

If the existence of the symmetry is unwanted, or the developer is unaware, this restriction may lead to unintended behaviors since not the entire argument domain will be explored. However, this behavior is often beneficial, because this restrictions can be exploited and can lead to significant speed-ups in calculations without loss of generality.