# Utilities

## Macros

`LazySets.@neutral`

— Macro`@neutral(SET, NEUT)`

Create methods to make a lazy set operation commutative with a given neutral-element set type.

**Input**

`SET`

– set type of lazy operation`NEUT`

– set type of neutral element

**Output**

Nothing.

**Notes**

This macro generates four functions (possibly two more if `@absorbing`

has been used in advance, and possibly two or four more if `@declare_array_version`

has been used in advance).

**Examples**

`@neutral(MinkowskiSum, N)`

creates at least the following methods:

`neutral(::MinkowskiSum) = N`

`MinkowskiSum(X, N) = X`

`MinkowskiSum(N, X) = X`

`MinkowskiSum(N, N) = N`

`LazySets.@absorbing`

— Macro`@absorbing(SET, ABS)`

Create methods to make a lazy set operation commutative with a given absorbing-element set type.

**Input**

`SET`

– set type of lazy operation`ABS`

– set type of absorbing element

**Output**

Nothing.

**Notes**

This macro generates four functions (possibly two more if `@neutral`

has been used in advance, and possibly two or four more if `@declare_array_version`

has been used in advance).

**Examples**

`@absorbing(MinkowskiSum, A)`

creates at least the following methods:

`absorbing(::MinkowskiSum) = A`

`MinkowskiSum(X, A) = A`

`MinkowskiSum(A, X) = A`

`MinkowskiSum(A, A) = A`

`LazySets.@neutral_absorbing`

— Macro`@neutral_absorbing(SET, NEUT, ABS)`

Create two methods to avoid method ambiguities for a lazy set operation with respect to neutral-element and absorbing-element set types.

**Input**

`SET`

– set type of lazy operation`NEUT`

– set type of neutral element`ABS`

– set type of absorbing element

**Output**

A quoted expression containing the function definitions.

**Notes**

This macro is used internally in other macros.

**Examples**

`@neutral_absorbing(MinkowskiSum, N, A)`

creates the following methods as quoted expressions:

`MinkowskiSum(N, A) = A`

`MinkowskiSum(A, N) = A`

`LazySets.@declare_binary_operation`

— Macro`@declare_binary_operation(SET)`

Create common methods for binary set operations.

**Input**

`SET`

– set type of the lazy operation

**Output**

Nothing.

**Notes**

This macro generates seven methods. See the example below.

**Examples**

`@declare_binary_operation(MinkowskiSum)`

creates the following methods:

`iterate(::MinkowskiSum)`

`length(::MinkowskiSum)`

`getindex(::MinkowskiSum, ::Int)`

`getindex(::MinkowskiSum, ::AbstractVector{Int})`

`lastindex(::MinkowskiSum)`

`array(::MinkowskiSum)`

`is_array_constructor(::Type{MinkowskiSum})`

`LazySets.@declare_array_version`

— Macro`@declare_array_version(SET, SETARR)`

Create methods to connect a lazy set operation with its array set type.

**Input**

`SET`

– set type of lazy operation`SETARR`

– set type of array version

**Output**

Nothing.

**Notes**

This macro generates six methods (and possibly up to eight more if `@neutral`

/`@absorbing`

has been used in advance for the base and/or array set type). See the example below.

**Examples**

`@declare_array_version(MinkowskiSum, MinkowskiSumArray)`

creates at least the following methods:

`array_constructor(::Type{MinkowskiSum}) = MinkowskiSumArray`

`binary_constructor(::Type{MinkowskiSumArray}) = MinkowskiSum`

`is_array_constructor(::Type{MinkowskiSumArray}) = true`

`MinkowskiSum!(X, Y)`

`MinkowskiSum!(X, arr)`

`MinkowskiSum!(arr, X)`

`MinkowskiSum!(arr1, arr2)`

`LazySets.@array_neutral`

— Macro`@array_neutral(FUN, NEUT, SETARR)`

Create two methods to avoid method ambiguities for a lazy set operation with respect to the neutral-element set type and the array set type.

**Input**

`FUN`

– function name`NEUT`

– set type of neutral element`SETARR`

– set type of array version

**Output**

A quoted expression containing the function definitions.

**Examples**

`@array_neutral(MinkowskiSum, N, ARR)`

creates the following methods as quoted expressions:

`MinkowskiSum(N, ARR) = ARR`

`MinkowskiSum(ARR, N) = ARR`

`LazySets.@array_absorbing`

— Macro`@array_absorbing(FUN, ABS, SETARR)`

Create two methods to avoid method ambiguities for a lazy set operation with respect to the absorbing-element set type and the array set type.

**Input**

`FUN`

– function name`ABS`

– set type of absorbing element`SETARR`

– set type of array version

**Output**

A quoted expression containing the function definitions.

**Examples**

`@array_absorbing(MinkowskiSum, ABS, ARR)`

creates the following methods as quoted expressions:

`MinkowskiSum(ABS, ARR) = ABS`

`MinkowskiSum(ARR, ABS) = ABS`

## Types

`LazySets.CachedPair`

— Type`CachedPair{N}`

A mutable pair of an index and a vector.

**Fields**

`idx`

– index`vec`

– vector

## Inspection of set interfaces

`LazySets.implementing_sets`

— Function```
implementing_sets(op::Function;
signature::Tuple{Vector{Type}, Int}=(Type[], 1),
type_args=Float64, binary::Bool=false)
```

Compute a dictionary containing information about availability of (unary or binary) concrete set operations.

**Input**

`op`

– set operation (respectively its`Function`

object)`signature`

– (optional, default:`Type[]`

) the type signature of the function without the`LazySet`

type(s) (see also the`index`

option and the`Examples`

section below)`index`

– (optional, default:`1`

) index of the set type in the signature in the unary case (see the`binary`

option)`type_args`

– (optional, default:`Float64`

) type arguments added to the`LazySet`

(s) when searching for available methods; valid inputs are a type or`nothing`

, and in the unary case (see the`binary`

option) it can also be a list of types`binary`

– (optional, default:`false`

) flag indicating whether`op`

is a binary function (`true`

) or a unary function (`false`

)

**Output**

A dictionary with three keys each mapping to a list:

`"available"`

– This list contains all set types such that there exists an implementation of`op`

.`"missing"`

– This list contains all set types such that there does not exist an implementation of`op`

. Note that this is the complement of the`"available"`

list.`"specific"`

– This list contains all set types such that there exists a type-specific implementation. Note that those set types also occur in the`"available"`

list.

In the unary case, the lists contain set types. In the binary case, the lists contain pairs of set types.

**Examples**

`shape_matrix`

is only available for ellipsoids.

```
julia> using LazySets: implementing_sets
julia> dict = implementing_sets(shape_matrix);
julia> dict["available"]
1-element Vector{Type}:
Ellipsoid
```

Every convex set type implements the function `σ`

.

```
julia> dict = implementing_sets(σ; signature=Type[AbstractVector], index=2);
julia> dict["missing"]
5-element Vector{Type}:
Complement
DensePolynomialZonotope
QuadraticMap
SimpleSparsePolynomialZonotope
SparsePolynomialZonotope
```

Some operations are not available for sets with rational numbers.

```
julia> N = Rational{Int};
julia> dict = implementing_sets(σ; signature=Type[AbstractVector{N}], index=2, type_args=N);
julia> Ball2 ∈ dict["missing"]
true
```

For binary functions, the dictionary contains pairs of set types. This check takes several seconds because it considers all possible set-type combinations.

```
julia> dict = LazySets.implementing_sets(convex_hull; binary=true);
julia> (HPolytope, HPolytope) ∈ dict["available"]
true
```

## File formats

`LazySets.read_gen`

— Method`read_gen(filename::String)`

Read a sequence of polygons stored in vertex representation (gen format).

**Input**

`filename`

– path of the file containing the polygons

**Output**

A list of polygons in vertex representation.

**Notes**

The `x`

and `y`

coordinates of each vertex should be separated by an empty space and polygons are separated by empty lines (even the last polygon). For example:

```
1.01 1.01
0.99 1.01
0.99 0.99
1.01 0.99
0.908463 1.31047
0.873089 1.31047
0.873089 1.28452
0.908463 1.28452
```

This is parsed as

```
2-element Array{VPolygon{Float64, Vector{Float64}},1}:
VPolygon{Float64, Vector{Float64}}([[1.01, 1.01], [0.99, 1.01], [0.99, 0.99], [1.01, 0.99]])
VPolygon{Float64, Vector{Float64}}([[0.908463, 1.31047], [0.873089, 1.31047], [0.873089, 1.28452], [0.908463, 1.28452]])
```

## Sampling

`LazySets._sample_unit_nsphere_muller!`

— Function```
_sample_unit_nsphere_muller!(D::Vector{Vector{N}}, n::Int, p::Int;
[rng]::AbstractRNG=GLOBAL_RNG,
[seed]::Union{Int, Nothing}=nothing) where {N}
```

Draw samples from a uniform distribution on an $n$-dimensional unit sphere using Muller's method.

**Input**

`D`

– output, vector of points`n`

– dimension of the sphere`p`

– number of random samples`rng`

– (optional, default:`GLOBAL_RNG`

) random number generator`seed`

– (optional, default:`nothing`

) seed for reseeding

**Output**

The modified vector `D`

.

**Algorithm**

This function implements Muller's method of normalized Gaussians [1] to uniformly sample over the $n$-dimensional sphere $S^n$ (which is the bounding surface of the $n$-dimensional unit ball).

Given $n$ canonical Gaussian random variables $Z₁, Z₂, …, Z_n$, the distribution of the vectors

\[\dfrac{1}{α}\left(z₁, z₂, …, z_n\right)^T,\]

where $α := \sqrt{z₁² + z₂² + … + z_n²}$, is uniform over $S^n$.

[1] Muller, Mervin E. *A note on a method for generating points uniformly on n-dimensional spheres.* Communications of the ACM 2.4 (1959): 19-20.

`LazySets._sample_unit_nball_muller!`

— Function```
_sample_unit_nball_muller!(D::Vector{Vector{N}}, n::Int, p::Int;
[rng]::AbstractRNG=GLOBAL_RNG,
[seed]::Union{Int, Nothing}=nothing) where {N}
```

Draw samples from a uniform distribution on an $n$-dimensional unit ball using Muller's method.

**Input**

`D`

– output, vector of points`n`

– dimension of the ball`p`

– number of random samples`rng`

– (optional, default:`GLOBAL_RNG`

) random number generator`seed`

– (optional, default:`nothing`

) seed for reseeding

**Output**

The modified vector `D`

.

**Algorithm**

This function implements Muller's method of normalized Gaussians [1] to uniformly sample from the interior of the unit ball.

Given $n$ Gaussian random variables $Z₁, Z₂, …, Z_n$ and a uniformly distributed random variable $r$ with support in $[0, 1]$, the distribution of the vectors

\[\dfrac{r^{1/n}}{α} \left(z₁, z₂, …, z_n\right)^T,\]

where $α := \sqrt{z₁² + z₂² + … + z_n²}$, is uniform over the $n$-dimensional unit ball.

[1] Muller, Mervin E. *A note on a method for generating points uniformly on n-dimensional spheres.* Communications of the ACM 2.4 (1959): 19-20.

`LazySets.sample`

— Function```
sample(B::Ball2{N}, [nsamples]::Int;
[rng]::AbstractRNG=GLOBAL_RNG,
[seed]::Union{Int, Nothing}=nothing) where {N}
```

Return samples from a uniform distribution on the given ball in the 2-norm.

**Input**

`B`

– ball in the 2-norm`nsamples`

– number of random samples`rng`

– (optional, default:`GLOBAL_RNG`

) random number generator`seed`

– (optional, default:`nothing`

) seed for reseeding

**Output**

A linear array of `nsamples`

elements drawn from a uniform distribution in `B`

.

**Algorithm**

Random sampling with uniform distribution in `B`

is computed using Muller's method of normalized Gaussians. This function requires the package `Distributions`

. See `_sample_unit_nball_muller!`

for implementation details.

```
sample(X::LazySet{N}, num_samples::Int;
[sampler]=_default_sampler(X),
[rng]::AbstractRNG=GLOBAL_RNG,
[seed]::Union{Int, Nothing}=nothing,
[include_vertices]=false,
[VN]=Vector{N}) where {N}
```

Random sampling of an arbitrary set `X`

.

**Input**

`X`

– set to be sampled`num_samples`

– number of random samples`sampler`

– (optional, default:`_default_sampler(X)`

) the sampler used; falls back to`CombinedSampler`

`rng`

– (optional, default:`GLOBAL_RNG`

) random number generator`seed`

– (optional, default:`nothing`

) seed for reseeding`include_vertices`

– (optional, default:`false`

) option to include the vertices of`X`

`VN`

– (optional, default:`Vector{N}`

) vector type of the sampled points

**Output**

A vector of `num_samples`

vectors. If `num_samples`

is not passed, the result is just one sample (not wrapped in a vector).

**Algorithm**

See the documentation of the respective `Sampler`

.

**Notes**

If `include_vertices == true`

, we include all vertices computed with `vertices`

. Alternatively if a number $k$ is passed, we plot the first $k$ vertices returned by `vertices(X)`

.

`LazySets.AbstractSampler`

— Type`AbstractSampler`

Abstract type for defining new sampling methods.

**Notes**

All subtypes should implement a `sample!(D, X, ::Method)`

method where the first argument is the output (vector of vectors), the second argument is the set to be sampled, and the third argument is the sampler instance.

`LazySets.CombinedSampler`

— Type`CombinedSampler <: AbstractSampler`

Type used for sampling arbitrary sets by trying different sampling strategies.

**Algorithm**

The algorithm is to first try a `RejectionSampler`

100 times. If that fails, it tries a `RandomWalkSampler`

.

`LazySets.FaceSampler`

— Type`FaceSampler <: AbstractSampler`

Type used for sampling from the `k`

-faces of a set.

**Fields**

`dim`

– dimension of the faces to be sampled; a negative number is interpreted as`n - dim`

where`n`

is the dimension of the set

**Notes**

For a three-dimensional polytope, the following face dimensions exist:

- 3-face – the polytope itself
- 2-faces – 2-dimensional polygonal faces
- 1-faces – 1-dimensional edges
- 0-faces – 0-dimensional vertices

For more information see Wikipedia.

**Algorithm**

Currently only hyperrectangles are supported. For each point to be sampled, we randomly split the integers `1 .. n`

into two subgroups of size `k`

and `n-k`

respectively. For the i-th coordinate in the first group, we sample in the interval `low(H, i) .. high(H, i)`

. For the i-th coordinate in the second group, we randomly pick either `low(H, i)`

or `high(H, i)`

.

`LazySets.HalfSpaceSampler`

— Type`HalfSpaceSampler{D} <: AbstractSampler`

Type used for sampling from a half-space.

**Fields**

`distribution`

– (optional, default:`nothing`

) distribution from which samples are drawn

**Notes**

If `distribution`

is `nothing`

(default), the sampling algorithm uses a `DefaultUniform`

over $[0, 1]^n$.

`LazySets.HyperplaneSampler`

— Type`HyperplaneSampler{D} <: AbstractSampler`

Type used for sampling from a hyperplane.

**Fields**

`distribution`

– (optional, default:`nothing`

) distribution from which samples are drawn

**Notes**

If `distribution`

is `nothing`

(default), the sampling algorithm uses a `DefaultUniform`

over $[0, 1]^n$.

`LazySets.SingletonSampler`

— Type`SingletonSampler <: AbstractSampler`

Type used for sampling from a singleton.

`LazySets.RejectionSampler`

— Type`RejectionSampler{D} <: AbstractSampler`

Type used for rejection sampling of a bounded set `X`

.

**Fields**

`distribution`

– (optional, default:`DefaultUniform`

) distribution from which the sample is drawn`tight`

– (optional, default:`false`

) set to`true`

if the support of the distribution is known to coincide with the set`X`

`maxiter`

– (optional, default:`Inf`

) maximum number of iterations before giving up

**Algorithm**

Draw a sample $x$ from a given distribution of a box-overapproximation of the original set $X$ in all $n$ dimensions. The function rejects a drawn sample $x$ and redraws as long as the sample is not contained in the original set $X$, i.e., while $x ∉ X$.

**Notes**

The `maxiter`

parameter is useful when sampling from sets that are small compared to their box approximation, e.g., flat sets, for which the probability of sampling from within the set is close to zero.

`LazySets.RandomWalkSampler`

— Type`RandomWalkSampler <: AbstractSampler`

Type used for sampling from a convex polytope using its vertex representation. This is especially useful if rejection sampling does not work because the polytope is flat.

**Fields**

`variant`

– (optional, default:`true`

) choice of a variant (see below)

**Notes**

The sampling is not uniform - points in the center of the polytope are more likely to be sampled.

The set to be sampled from must provide its vertices via `vertices_list`

.

**Algorithm**

Choose a random convex combination of the vertices of a convex polytope `X`

.

If `variant == false`

, we proceed as follows. Let $V = \{v_i\}_i$ denote the set of vertices of `X`

. Then any point $p \in \mathbb{R}^n$ of the convex polytope $X$ is a convex combination of its vertices, i.e., $p = \sum_{i} v_i α_i$ for some (non-negative) coefficients $\{α_i\}_i$ that add up to 1. The algorithm chooses a random convex combination (the $α_i$). To produce this combination, we apply the finite-difference operator on a sorted uniform sample over $[0, 1]$; the method can be found in [1] and [2].

If `variant == true`

, we start from a random vertex and then repeatedly walk toward a random vertex inside the polytope.

**References**

[1] *Rubin, Donald B. The Bayesian bootstrap. The annals of statistics (1981): 130-134.*

`LazySets.PolynomialZonotopeSampler`

— Type`PolynomialZonotopeSampler{D} <: AbstractSampler`

Type used for sampling from polynomial zonotopes.

**Fields**

`distribution`

– (optional, default:`nothing`

) distribution from which samples are drawn

**Notes**

If `distribution`

is `nothing`

(default), the sampling algorithm uses a `DefaultUniform`

over $[-1, 1]^n$.

## Symbolics

`LazySets._vec`

— Function`_vec(vars)`

Transform a tuple of operations into one vector of operations.

**Input**

`vars`

– tuple where each element is either variable-like (`Num`

) or a vector of variables (`Vector{Num}`

)

**Output**

A vector of `Operation`

obtained by concatenating each tuple component.

**Examples**

```
julia> using Symbolics
julia> vars = @variables x[1:2] y
2-element Vector{Any}:
x[1:2]
y
julia> LazySets._vec(vars)
3-element Vector{Num}:
x[1]
x[2]
y
```

## Functions for numbers

`LazySets.sign_cadlag`

— Function`sign_cadlag(x::Real)`

This function works like the sign function but is $1$ for input $0$.

**Input**

`x`

– real scalar

**Output**

$1$ if $x ≥ 0$, $-1$ otherwise.

**Notes**

This is the sign function right-continuous at zero (see càdlàg function). It can be used with vector-valued arguments via the dot operator.

**Examples**

```
julia> LazySets.sign_cadlag.([-0.6, 1.3, 0.0])
3-element Vector{Float64}:
-1.0
1.0
1.0
```

`LazySets.minmax`

— Function`minmax(a, b, c)`

Compute the minimum and maximum of three numbers a, b, c.

**Input**

`a`

– number`b`

– number`c`

– number

**Output**

The minimum and maximum of three given numbers.

**Examples**

```
julia> LazySets.minmax(1.4, 52.4, -5.2)
(-5.2, 52.4)
```

`LazySets.arg_minmax`

— Function`arg_minmax(a, b, c)`

Compute the indices of the minimum and maximum of three numbers a, b, c.

**Input**

`a`

– first number`b`

– second number`c`

– third number

**Output**

The indices of the minimum and maximum of the three given numbers.

**Examples**

```
julia> LazySets.arg_minmax(1.4, 52.4, -5.2)
(3, 2)
```

## Other functions

`LazySets._an_element_helper_hyperplane`

— Function```
_an_element_helper_hyperplane(a::AbstractVector{N}, b,
[nonzero_entry_a]::Int) where {N}
```

Helper function that computes an element on a hyperplane $a⋅x = b$.

**Input**

`a`

– normal direction`b`

– constraint`nonzero_entry_a`

– (optional, default: computes the first index) index`i`

such that`a[i]`

is different from 0

**Output**

An element on a hyperplane.

**Algorithm**

We compute the point on the hyperplane as follows:

- We already found a nonzero entry of $a$ in dimension, say, $i$.
- We set $x[i] = b / a[i]$.
- We set $x[j] = 0$ for all $j ≠ i$.

`LazySets.binary_search_constraints`

— Function```
binary_search_constraints(d::AbstractVector{N},
constraints::Vector{<:HalfSpace{N}};
[start_index]::Int=div(length(constraints)+1, 2),
[choose_lower]::Bool=false) where {N}
```

Perform a binary search in the constraints.

**Input**

`d`

– direction`constraints`

– constraints`start_index`

– (optional, default:`div(length(constraints)+1, 2)`

) start index`choose_lower`

– (optional, default:`false`

) flag for choosing the lower index (see the 'Output' section)

**Output**

In the default setting, the result is the smallest index `k`

such that `d <= constraints[k].a`

, or `length(constraints)+1`

if no such `k`

exists. If the `choose_lower`

flag is set, the result is the largest index `k`

such that `constraints[k].a < d`

, which is equivalent to being `k-1`

in the normal setting.

`LazySets.get_constrained_lowdimset`

— Function```
get_constrained_lowdimset(cpa::CartesianProductArray{N, S},
P::AbstractPolyhedron{N}) where {N, S}
```

Preprocessing step for the intersection between a Cartesian product of a finite number of sets and a polyhedron.

**Input**

`cpa`

– Cartesian product of a finite number of sets`P`

– polyhedron

**Output**

A four-tuple of:

- a low-dimensional
`CartesianProductArray`

in the constrained dimensions of the original set`cpa`

- the variables in the constrained blocks,
- the original block structure of the low-dimensional sets,
- the list of the constrained blocks.

`LazySets.get_radius!`

— Function`get_radius!(sih::SymmetricIntervalHull{N}, i::Int) where {N}`

Compute the radius of the symmetric interval hull of a set in a given dimension.

**Input**

`sih`

– symmetric interval hull of a set`i`

– dimension in which the radius should be computed

**Output**

The radius of the symmetric interval hull of a set in a given dimension.

**Algorithm**

We ask for the `extrema`

of the underlying set in dimension `i`

.

`LazySets.is_tighter_same_dir_2D`

— Function```
is_tighter_same_dir_2D(c1::HalfSpace,
c2::HalfSpace;
[strict]::Bool=false)
```

Check if the first of two two-dimensional constraints with equivalent normal direction is tighter.

**Input**

`c1`

– first linear constraint`c2`

– second linear constraint`strict`

– (optional; default:`false`

) check for strictly tighter constraints?

**Output**

`true`

iff the first constraint is tighter.

`LazySets._leq_trig`

— Function`_leq_trig(u::AbstractVector{N}, v::AbstractVector{N}) where {N<:AbstractFloat}`

Compare two 2D vectors by their direction.

**Input**

`u`

– first 2D direction`v`

– second 2D direction

**Output**

`true`

iff $\arg(u) [2π] ≤ \arg(v) [2π]$.

**Notes**

The argument is measured in counter-clockwise fashion, with the 0 being the direction (1, 0).

**Algorithm**

The implementation uses the arctangent function with sign, `atan`

, which for two arguments implements the `atan2`

function.

`LazySets.same_block_structure`

— Function```
same_block_structure(x::AbstractVector{S1}, y::AbstractVector{S2}
) where {S1<:LazySet, S2<:LazySet}
```

Check whether two vectors of sets have the same block structure, i.e., the $i$-th entry in the vectors have the same dimension.

**Input**

`x`

– first vector`y`

– second vector

**Output**

`true`

iff the vectors have the same block structure.

`LazySets._σ_hyperplane_halfspace`

— Function```
_σ_hyperplane_halfspace(d::AbstractVector, a, b;
[error_unbounded]::Bool=true,
[halfspace]::Bool=false)
```

Return a support vector of a hyperplane $a⋅x = b$ in direction `d`

.

**Input**

`d`

– direction`a`

– normal direction`b`

– constraint`error_unbounded`

– (optional, default:`true`

)`true`

if an error should be thrown whenever the set is unbounded in the given direction`halfspace`

– (optional, default:`false`

)`true`

if the support vector should be computed for a half-space

**Output**

A pair `(v, f)`

where `v`

is a vector and `f`

is a Boolean flag.

The flag `f`

is `false`

in one of the following cases:

- The direction has norm zero.
- The direction is (a multiple of) the hyperplane's normal direction.
- The direction is (a multiple of) the opposite of the hyperplane's normal

direction and `halfspace`

is `false`

. In all these cases, `v`

is any point on the hyperplane.

Otherwise, the flag `f`

is `true`

, the set is unbounded in the given direction, and `v`

is any vector.

If `error_unbounded`

is `true`

and the set is unbounded in the given direction, this function throws an error instead of returning.

**Notes**

For correctness, consider the weak duality of LPs: If the primal is unbounded, then the dual is infeasible. Since there is only a single constraint, the feasible set of the dual problem is $a ⋅ y == d$, $y ≥ 0$ (with objective function $b ⋅ y$). It is easy to see that this problem is infeasible whenever $a$ is not parallel to $d$.