# Convolution Is Fancy Multiplication

Savor making engineering faculty college students squirm? Receive them phrase convolution and (would possibly perchance perchance soak up to you would possibly perchance perchance presumably presumably even be barbarous) the convolution theorem. They'll clarify one factor about sliding home windows as they attempt to coast through one.

Convolution is on the overall provided with its formal definition: Yikes. Let's begin with out calculus: Convolution is esteem multiplication.

## Allotment 1: Well being facility Analogy

Factor in you convey up a hospital treating victims with a single sickness. You would possibly perchance perchance presumably presumably even soak up:

• A remedy perception: `` Every affected person will get Three items of the treatment on their first day.
• An stock of victims: `[1 2 3 4 5]` Your affected person agenda for the week (1 explicit individual Monday, 2 on Tuesday, and much others.).

Are ready for: How appreciable drugs assemble you verbalize day after day? Well, that's factual a fleet multiplication:

``````Arrangement  Sufferers      =Day-to-day Usage
   [1 2 3 4 5]   =[3 6 9 12 15]
``````

Multiplying the idea by the affected person guidelines gives the utilization for the upcoming days: `[3 6 9 12 15]`. Every single day multiplication (`Three x 4`) method the utilization of the idea with a single day of victims: ` =`.

### Intuition For Convolution

As an occasion the sickness mutates and requires a multi-day remedy. You bear a model authentic perception: `Arrangement: [3 2 1]`

That method Three items of the treatment on essentially the most predominant day, 2 on the 2nd, and 1 on the third. Ample. Given the an identical affected person agenda ( `[1 2 3 4 5]`), what's our drugs utilization day after day?

Uh... shoot. Or not it isn't a fleet multiplication:

• On Monday, 1 affected person is obtainable in. Or not it is her first day, so she will be able to get Three items.
• On Tuesday, the Monday gal will get 2 items (her 2nd day), however two authentic victims close to, who salvage Three each (2 3=6). The full is 2 + (2 3)=Eight items.
• On Wednesday, or not it is trickier: The Monday gal finishes (1 unit, her remaining day), the Tuesday of us salvage 2 items (2 2), and there are Three authentic Wednesday of us... argh.

The victims are overlapping and or not it isn't simple to hint. How assemble we convey up this calculation?

An thought: think about flipping the affected person guidelines, so essentially the most predominant affected person is on the ethical:

``````           Commence of line
5 Four Three 2 1
``````

Next, think about we soak up Three separate rooms the place we apply the ethical dose:

`````` Rooms     Three 2 1
``````

On your first day, you stroll into essentially the most predominant room and salvage Three items of drugs. The following day, it is important into room #2 and salvage 2 items. On the ultimate day, you stroll into room #Three and salvage 1 unit. There's no rooms afterwards, and in addition you would possibly perchance perchance presumably presumably even be executed.

To calculate the overall drugs utilization, line up the victims and stroll them through the rooms:

`````` Monday
----------------------------
Rooms                  Three 2 1
Sufferers       5 Four Three 2 1

Usage                  3
``````

On Monday (our first day), we soak up a single affected person inside essentially the most predominant room. She will get Three items, for a complete utilization of three. Makes sense, ethical?

On Tuesday, all people takes a step ahead:

`````` Tuesday
----------------------------
Rooms                  Three 2 1
Sufferers ->      5 Four Three 2 1

Usage                  6 2     =8
``````

The first affected person is now throughout the 2nd room, and there is 2 authentic victims inside essentially the most predominant room. We multiply each room's dose by the affected person rely, then combine.

Every single day we factual stroll the guidelines ahead:

`````` Wednesday
----------------------------
Rooms                  Three 2 1
Sufferers ->        5 Four Three 2 1

Usage                  9 Four 1   =14

Thursday
-----------------------------
Rooms                  Three 2 1
Sufferers ->          5 Four Three 2 1

Usage                 12 6 2   =20

Friday
-----------------------------
Rooms                  Three 2 1
Sufferers ->            5 Four Three 2 1

Usage                 15 Eight 3   =26
``````

Whoa! Or not it is intricate, however we figured it out, ethical? We are capable of fetch the utilization for any day by reversing the guidelines, sliding it to the required day, and mixing the doses.

The full day-by-day utilization seems to be love this (assemble not neglect Sat and Sun, since some victims started on Friday):

``````Arrangement      Affected individual List  =Whole Day-to-day Usage

[3 2 1]   [1 2 3 4 5]   =[3 8 14 20 26 14 5]
M T W T F        M T W  T  F  S  S
``````

This calculation is the convolution of the idea and affected person guidelines. Or not it is a esteem multiplication between a convey of a numbers and a "program".

### Interactive Demo

Here's a live demo. Are attempting altering `F` (the idea) or `G` (the affected person guidelines). The convolution \$c(t)\$ matches our handbook calculation above.

(We make clear useful properties \$f(x)\$ and \$g(x)\$ to pad each guidelines with zero, and modify for the guidelines index beginning up at 1).

You would possibly perchance perchance presumably presumably additionally assemble a fleet convolution with Wolfram Alpha:

``````ListConvolve[{3, 2, 1}, {1, 2, 3, 4, 5}, {1, -1}, 0]
{3, 8, 14, 20, 26, 14, 5}
``````

(The additional `{1, -1}, 0` aligns the lists and pads with zero.)

### Utility: COVID Ventilator Usage

I began this text 5 years in the past (instinct takes some time...), however sadly the analogy is related to this present day.

Let's make the most of convolution to estimate ventilator utilization for incoming victims.

• Space \$f(x)\$ because the p.c of victims needing ventilators. As an occasion, `[.05 .03 .01]` method 5% of victims want ventilators essentially the most predominant week, 3% the 2nd week, and 1% the third week.
• Space \$g(x)\$ because the weekly incoming victims, in 1000's.
• The convolution \$c(t)=f g\$, shows what variety of ventilators are needed every and per week (in 1000's). \$c(5)\$ is what variety of ventilators are needed 5 weeks from now.

Let's try it out:

• `F=[.05, .03, .01]` is the ventilator make the most of share by week
• `G=[10, 20, 30, 20, 10, 10, 10]`, is the incoming hospitalized victims. It begins at 10okay per week, rises to 30okay, then decays to 10okay.

With these numbers, we quiz a max ventilator make the most of of two.2k in 2 weeks: The convolution drops to Zero after 9 weeks given that affected person guidelines has toddle out. On this occasion, we're drawn to the top worth the convolution hits, not the prolonged-timeframe complete.

Other plans to convolve would possibly perchance perchance presumably additionally very successfully be drug doses, vaccine appointments (one this present day, each different a month from now), reinfections, and different superior interactions.

The hospital analogy is the psychological model I want I had when discovering out. Now that we soak up tried it with proper numbers, let's pour throughout the Math Juice and flip the analogy into calculus.

## Allotment 2: The Calculus Definition

So, what happened in our occasion? We had a guidelines of victims and a perception. If the idea had been easy (single day ``), odd multiplication would soak up labored. For the explanation that perception was superior, we wanted to "convolve" it.

Time for some Fun Information&commerce;:

• Convolution is written \$f g\$, with an asterisk. Certain, an asterisk on the overall signifies multiplciation, however in developed calculus class, it signifies a convolution. Habitual multiplication is factual implied (\$fg\$).

• The outcomes of a convolution is a model authentic attribute that gives the overall utilization for any day ("What was the overall utilization on day \$t=3\$?"). We are capable of graph the convolution over time to stare the day-by-day totals.

Now the immense aha: Convolution reverses regarded as one of many essential lists! Here's why.

Let's name our remedy perception \$f(x)\$. In our occasion, we mild `[3 2 1]`.

The guidelines of victims (inputs) is \$g(x)\$. Alternatively, we should reverse this guidelines as we breeze it, so the earliest affected person (Monday) enters the hospital first (first in, first out). This means we should make the most of \$g(-x)\$, the horizontal reflection of \$g(x)\$. `[1 2 3 4 5]` turns into `[5 4 3 2 1]`.

Now that we soak up the reversed guidelines, select a day to compute (\$t=0, 1, 2...\$). To maneuver our affected person guidelines by this appreciable, we make the most of: \$g(-x + t)\$. That's, we reverse the guidelines (\$-x\$) and bounce to the pleasing day (\$+t\$).

We've our situation:

• \$f(x)\$ is the idea to make the most of
• \$g(-x + t)\$ is the guidelines of inputs (flipped and slid to the ethical day).

To salvage the overall utilization on day \$t\$, we multiply each affected person with the idea, and sum the outcomes (an integral). To fable for any that you simply would possibly perchance perchance presumably presumably additionally like of dimension, we trek from -infinity to +infinity.

Now we are able to describe convolution formally the utilization of calculus: Phew! That's fairly few symbols. Some notes:

• We make the most of a dummy variable \$tau\$ (tau) for the intermediate computation. Factor in \$tau\$ as knocking on each room (\$tau={0, 1, 2, 3...}\$), discovering the dosage [\$f(tau)\$], the completely different of victims [\$g(t - tau)\$], multiplying them, and totaling issues throughout the integral. Yowza. The so-called "dummy" variable \$tau\$ is love `i` in a `for` loop: or not it is short-timeframe, however does the work. (By analogy, \$t\$ is a world variable has a mounted worth all through the loop.)
• In the obliging definition, you'll peep \$g(t - tau)\$ fairly than \$g(- tau+ t)\$. The 2nd mannequin shows the flip (\$-tau\$) and breeze (\$+t\$). Writing \$g(t - tau)\$ makes it seem love we're drawn to the variation between the variables, which confused me.
• The remedy perception (program to toddle) is typically referred to as the kernel: you convolve a kernel with an enter.

No longer too imperfect, ethical? The equation is a correct description of the analogy.

## Allotment 3: Mathematical Properties of Convolution

We cannot stare a model authentic math operation with out taking it for a trek. Let's peep the blueprint through which it behaves.

### Convolution is commutative: f g=g f

In our computation, we flipped the affected person guidelines and saved the idea the an identical. Can additionally we soak up flipped the idea as a substitute?

You wager. Negate referring to the victims are motionless, and deal with of their rooms: `[1 2 3 4 5]`. To dispute the drugs, we soak up Three scientific carts that trek to each room and dispute the dose. Every single day, they breeze ahead one arrange.

``````  Carts ->
1 2 3

1 2 Three Four 5
Sufferers
``````

As ahead of, even though our perception is written `[3 2 1]` (Three items on essentially the most predominant day), we flip the painting of the carts to`[1 2 3]`. That method, a affected person will get Three items on their first day, as we quiz. Checking with Wolfram Alpha, the calculation is the an identical.

``````ListConvolve[{1, 2, 3, 4, 5}, {3, 2, 1}, {1, -1}, 0]
{3, 8, 14, 20, 26, 14, 5}
``````

Chilly! It seems to be love convolution is commutative: and we are able to come to a dedication to flip each \$f\$ or \$g\$ when calculating the integral. Reliable, ethical?

### The integral of the convolution

When all remedies are accomplished, what was the complete drugs utilization? This is the integral of the convolution. (A jiffy in the past, that phrase would soak up you ever leaping out of a window.)

But it little doubt's a simple calculation. Our perception gives each affected person `sum([3 2 1])=6` items of drugs. And we soak up `sum([1 2 3 4 5])=15` victims. The full utilization is factual `6 x 15=90` items.

Wow, that was easy: the utilization for the full convolution is factual the product of the subtotals! I hope this clicks intuitively. Existing that this trick works for convolution, however not integrals in complete. As an occasion: If we separate \$x cdot x\$ into two integrals we salvage:

and people do not seem like the an identical. (Calculus can be appreciable extra easy if we'd perchance perchance presumably additionally shatter up aside integrals love this.) Or not it is outlandish, however \$int (f g)\$ would possibly perchance perchance be additional helpful to determine than \$int (fg)\$.

### Impulse Response

What occurs if we despatched a single affected person through the hospital? The convolution would factual be that day's perception.

``````Arrangement    Sufferers=Convolution
[3 2 1]      =[3 2 1]
``````

In different phrases, convolving with `` gives us the distinctive perception.

In calculus phrases, a spike of `` (and nil in each different case) is the Dirac Delta Characteristic. By method of convolutions, this attribute acts love the #1 and returns the distinctive attribute: We are capable of extend the delta attribute by T, which delays the following convolution attribute too. Factor in a affected person shows up per week uninteresting, so no drugs us mild for per week: ## Allotment 4: Convolution Theorem & The Fourier Turn out to be

The Fourier Turn out to be (written with a esteem \$mathscr{F}\$) converts a attribute \$f(t)\$ right into a guidelines of cyclical substances \$F(s)\$: As an operator, it would be written \$mathscr{F}lbrace f rbrace=F\$.

In our analogy, we convolved the idea and affected person guidelines with a esteem multiplication. For the explanation that Fourier Turn out to be gives us lists of substances, would possibly perchance perchance presumably additionally we salvage the an identical end result by mixing the ingredient lists?

Yep, we are able to: Cherish multiplication throughout the odd world is odd multiplication throughout the esteem world.

In math phrases, "Convolution throughout the time area is multiplication throughout the frequency (Fourier) area."

Mathematically, right here is written: or the place \$f(x)\$ and \$g(x)\$ are useful properties to convolve, with transforms \$F(s)\$ and \$G(s)\$.

We are capable of painting this theorem with developed calculus, that makes use of theorems I assemble not fairly model, however let's like through the that method.

Because of \$F(s)\$ is the Fourier Turn out to be of \$f(t)\$, we are able to question for a specific frequency (\$s=2textual relate materials{Hz}\$) and salvage the mixed interplay of each and each knowledge degree with that frequency. Let's recount: That method after each knowledge degree has been multiplied towards the twoHz cycle, the stop end result's \$3 + i\$. But we'd perchance perchance presumably even soak up saved each interplay separate: Where \$c_t\$ is the contribution to the twoHz frequency from datapoint \$t\$. In an an identical sort, we are able to fabricate greater \$G(s)\$ right into a guidelines of interactions with the twoHz ingredient. Let's recount \$G(2)=7 - i\$: The Convolution Theorem is de facto asserting: Our convolution throughout the odd area entails fairly a few rotten-multiplications. In the esteem frequency area, we soundless soak up a bunch of interactions, however \$F(s)\$ and \$G(s)\$ soak up consolidated them. We are capable of factual multiply \$F(2)G(2)=(3 + i)(7-i)\$ to fetch the twoHz ingredient throughout the convolved end result.

By analogy, recount you might be looking out to calculate: Or not it is fairly a few labor to rotten-multiply each timeframe: \$(1 cdot 5) + (1cdot 6) + (1cdot 7) + ...\$

Or not it is higher to consolidate the teams into \$(1 + 2 + 3 + 4)=10\$ and \$(5 + 6 + 7 + 8)=26\$, and then multiply to salvage \$10 cdot 26=260\$.

This nuance precipitated me fairly a few confusion. It seems love \$FG\$ is a single multiplication, whereas \$f g\$ entails a bunch of intermediate phrases. I forgot that \$F\$ already did the work of merging a bunch of entries right into a single one.

Now, we do not seem like fairly executed. We are capable of convert \$f g\$ throughout the time area into \$FG\$ throughout the frequency area, however we doubtlessly want it once more throughout the time area for a usable end result: You would possibly perchance perchance presumably presumably even soak up a riddle in English (\$f g\$), you translate it to French (\$FG\$), salvage your shipshape French buddy to determine that calculation, then convert it once more to English (\$mathscr{F}^{-1}\$).

### And in reverse...

The convolution theorem works this manner too: Habitual multiplication throughout the odd world is esteem multiplication throughout the esteem world.

Chilly, eh? Rather then multiplying two useful properties love some cave dweller, arrange in your monocle, convolve the Fourier Transforms, and and convert to the time area: I'm not asserting right here is enjoyable, factual that or not it is that you simply would possibly perchance perchance presumably presumably additionally like of. If your French buddy has a gnarly calculation they're preventing, it will probably perchance presumably discover love arithmetic to you.

### Mini proof

Assign in ideas how we stated the integral of a convolution was a multiplication of the individual integrals? Well, the Fourier Turn out to be is factual a with out a doubt specific integral, ethical? So (handwaving), it seems we'd perchance perchance presumably additionally swap the total-reason integral \$int\$ for \$mathscr{F}\$ and salvage which is the convolution theorem. I want a deeper instinct for the proof, however this helps issues click on on.

## Allotment 5: Capabilities

The trick with convolution is discovering a substantial "program" (kernel) to make the most of to your enter. Here's a few examples.

### Shifting averages

As an occasion it is important a transferring common between neighboring objects in a guidelines. That's half of each and each aspect, added collectively: It is much a "multiplication program" of `[0.5 0.5]` convolved with our guidelines:

``````ListConvolve[{1, 4, 9, 16, 25}, {0.5, 0.5}, {1, -1}, 0]
{0.5, 2.5, 6.5, 12.5, 20.5, 12.5}
``````

We are capable of manufacture a transferring common with a single operation. Comely!

A 3-element transferring common can be `[.33 .33 .33]`, a weighted common would possibly perchance perchance presumably additionally very successfully be `[.5 .25 .25]`.

### Derivatives

The spinoff finds the variation between neighboring values. Here's the idea: `[1 -1]`

``````ListConvolve[{1, 2, 3, 4, 5}, {1, -1}, {1, -1}, 0]
{1, 1, 1, 1, 1, -5}                         // -5 since we ran out of entries

ListConvolve[{1, 4, 9, 16, 25}, {1, -1}, {1, -1}, 0]
{1, 3, 5, 7, 9, -25}                        // discrete spinoff is 2x + 1
``````

With a simple kernel, we are able to fetch a substantial math property on a discrete guidelines. And to salvage a 2nd spinoff, factual apply the spinoff convolution twice:

`F [1 -1] [1 -1]`

As a shortcut, we are able to precompute the ultimate convolutions (`[1 -1] [1 -1]` ) and salvage:

``````ListConvolve[{1, -1}, {1,-1}, {1, -1}, 0]
{1, -2, 1}
``````

Now we soak up a single kernel `[1, -2, 1]` that can get the 2nd spinoff of a guidelines:

``````ListConvolve[{1,4, 9, 16, 25}, {1, -2, 1}, {1, -1}, 0]
{1, 2, 2, 2, 2, -34, 25}
``````

Other than the boundary objects, we salvage the anticipated 2nd spinoff: ### Blurring / unblurring images

An picture blur is actually a convolution of your picture with some "blurring kernel": The blur of our 2D picture requires a 2D common: Can we undo the blur? Yep! With our buddy the Convolution Theorem, we are able to assemble:     Whoa! We are capable of enhance the distinctive picture by dividing out the blur. Convolution is a simple multiplication throughout the frequency area, and deconvolution is a simple division throughout the frequency area. A short while once more, the considered "deblurring by dividing Fourier Transforms" was gibberish to me. Whereas it would be daunting mathematically, or not it is getting additional helpful conceptually.

More studying:

• Wikipedia on deconvolution, deblurring.
• Image Restoration Lecture, each different one, and occasion for above
• Blind deconvolution is would possibly perchance perchance soak up to you assemble not know the blur kernel (fabricate a wager)

### Algorithm Trick: Multiplication

What's a bunch? An stock of digits:

``````1234=1000 + 200 + 30 + 4=[1000 200 30 4]
5678=5000 + 600 + 70 + 8=[5000 600 70 8]
``````

And what's odd, grade-college multiplication? A digit-by-digit convolution! We sweep one guidelines of digits by the alternative, multiplying and including as we trek: Supply

We are capable of manufacture the calculation by convolving the lists of digits (wolfram alpha):

``````ListConvolve[{1000, 200, 30, 4}, {8, 70, 600, 5000}, {1, -1}, 0]
{8000, 71600, 614240, 5122132, 1018280, 152400, 20000}

sum {8000, 71600, 614240, 5122132, 1018280, 152400, 20000}
7006652
``````

Existing that we pre-flip regarded as one of many essential lists (it is going to get swapped throughout the convolution later), and the intermediate calculations are a restricted completely completely different. But, combining the subtotals gives the anticipated end result.

### Faster Convolutions

Why convolve fairly than doing a odd digit-by-digit multiplication? Well, the convolution theorem lets us change convolution with Fourier Transforms: The convolution (\$f g\$) has complexity \$O(n^2)\$. We've \$n\$ positions to activity, with \$n\$ intermediate multiplications at each arrange.

The ethical facet entails:

• Two Fourier Transforms, which could perchance perchance presumably be on the overall \$O(n^2)\$. Alternatively, the Snappy Fourier Turn out to be (a divide-and-conquer talent) makes them \$O(nlog(n))\$.
• Pointwise multiplication of the results of the transforms (\$sum a_n cdot b_n\$), which is \$O(n)\$
• An inverse rework, which is \$O(nlog(n))\$

And the overall complexity is: \$O(nlog(n)) + O(nlog(n)) + O(n) + O(nlog(n))=O(nlog(n))\$

Habitual multiplication throughout the esteem area is sooner than a esteem multiplication throughout the odd area. Our French buddy is rarely any trot. (More)

### Convolutional Neural Nets (CNN)

Machine discovering out is prepared discovering the mathematics useful properties that rework enter knowledge right into a desired end result (a prediction, classification, and much others.).

Beginning with an enter sign, we'd perchance perchance presumably additionally convolve it with a bunch of kernels: Given that convolution can assemble superior math (transferring averages, blurs, derivatives...), it seems some mixture of kernels would possibly perchance perchance soak up to flip our enter into one factor appreciable, ethical?

Convolutional Neural Nets (CNNs) activity an enter with layers of kernels, optimizing their weights (plans) to achieve a goal. Factor in tweaking the remedy perception to deal with drugs utilization beneath some threshold.

CNNs are sometimes mild with picture classifiers, however 1D knowledge items work factual vibrant.

• Digit classifier demo: https://cs.stanford.edu/of us/karpathy/convnetjs/demo/mnist.html

### LTI Arrangement Habits

A linear, time-invariant association method:

• Linear: Scaling and mixing inputs scales and combines outputs by the an identical amount
• Time invariant: Outputs rely on relative time, not absolute time. You salvage Three items on your first day, and or not it isn't related if or not it is Wednesday or Thursday.

A esteem phrase is "A LTI association is characterised by its impulse response". Translation: If we ship a single affected person through the hospital ``, we'll stare the remedy perception. Then we are able to predict the utilization for any sequence of victims by convolving it with the idea. If the association will not be with out a doubt LTI, we won't extrapolate in line with a single explicit individual's abilities. Scaling the inputs would possibly perchance perchance presumably additionally not scale the outputs, and the fitting calendar day (not relative day) would possibly perchance perchance presumably additionally have an effect on the stop end result.

### Engineering Analogies

From David Greenspan: "Negate you soak up gotten a definite laser pointer that makes a celebrity type on the wall. You tape collectively a bunch of these laser pointers throughout the type of a sq.. The pattern on the wall now would possibly perchance perchance presumably be the convolution of a celebrity with a sq.."

Habitual multiplication gives you a single scaled reproduction of an enter. Convolution creates a pair of overlapping copies that bear in mind a pattern you soak up specified.

Genuine-world programs soak up squishy, not instantaneous, habits: they ramp up, high, and plunge down. The convolution lets us model programs that echo, reverb and overlap.

Now or not it is time for the distinguished sliding window occasion. Mediate of a pulse of inputs (crimson) sliding through a device (blue), and having a mixed enact (yellow): the convolution. (Supply)

## Summary

Convolution has an developed technical definition, nonetheless the fundamentals shall be understood with the ethical analogy.

Fleet rant: I take into fable math for enjoyable, nevertheless it took years to fetch a satisfying instinct for:

• Why is one attribute reversed?
• Why is convolution commutative?
• Why does the integral of the convolution=product of integrals?
• Why are the Fourier Transforms multiplied level-by-level, and never overlapped?

Why'd it take goodbye? Factor in discovering out multiplication with \$f occasions g=z\$ fairly than \$Three occasions 5=15\$. With out an occasion I'll discover in my head, I would perchance perchance presumably additionally handiest memorize outcomes, not intuit them. Expectantly this analogy can arrange you years of battle.

Overjoyed math.

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## Other Posts In This Series

1. A Visual, Intuitive Manual to Imaginary Numbers
2. Intuitive Arithmetic With Complex Numbers
3. Arrangement Why Complex Multiplication Works
4. Intuitive Manual to Angles, Levels and Radians
5. Intuitive Arrangement Of Euler's Arrangement
6. An Interactive Manual To The Fourier Turn out to be
7. Intuitive Manual to Convolution
8. Intuitive Arrangement of Sine Waves
9. An Intuitive Manual to Linear Algebra
10. A Programmer's Intuition for Matrix Multiplication
11. Imaginary Multiplication vs. Imaginary Exponents

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