# Optical Forums > Progressive Lens Discussion Forum >  Justifying unequal fitting heights

## TheRobotious

When measuring for PALs, does anyone 'split' the fitting heights or average the heights, rather than using measured unequal fitting heights? If so, can someone justify why they do this? Is there some risk involved with using unequal heights, assuming the frame is fully adjusted and sitting straight on the face? Admittedly I lack experience in this area, but I always take measurements 'by the book' whereas most all of my colleagues measure one eye only and use that measurement for both eyes. 

No one at work has really justified using equal heights, so is there something I'm missing? The way I see it, using unequal heights would only solve more problems compared with using equal 'fabricated' heights when the pupil heights are in fact not level. Any thoughts to the contrary?

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## mervinek

I have used unequal heights on occasion, but it's rare.  The most obvious time was with a patient who had cancer and had a huge (5mm+) asymmetry between the eyes. And I always measure both eyes.

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## Boldt

First how much of a split? .5mm-1mm go with the lower seg if you're not worried about it. 
For my self though I do split at 1mm, I'vbe found that it helps with new p[progressive users, and those with a wonky or high RX. Think about it like this, you want to have the segs fit at the pupil for the best use on progressives. When fit to low people have to raise their head to use the reading part. Now think about having one spot on and one to low. How messed up would that make the person's vision? Generally 1mm is ok, but I'd rather them be better than ok my self. 
I do the same with SV and have people loving it.
So I guess for me it's stepping up my own game and going that extra mile

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## theyeshaveit

You are doing it the right way.  90% of the population is asymmetrical so why would anyone assume the seg hts are the same?  Laziness in my opinion.  I always pre fit the frame and the measure seg hts on both eyes.  It takes a couple minutes longer, but is worth it for the benefit of the wearer.  Keep on doing it the right way.  Your customers will appreciate it in the long run.

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## edKENdance

What do you do for clients who could be wearing uneven segs but have been wearing aligned segs for years?

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## Boldt

I explain to them what I'm doing, and that it may seem off at first. I tell them to give it two weeks of constant use and If they don't like it still I'll swap them back to what they had. It's generally a lab redo that is at no charge. So no worries for both of us.

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## TheRobotious

Thanks very much for the insight everyone. Patients who come in having happily worn poorly measured glasses is always a bit tricky, but I agree with Boldt in terms of explaining exactly what you'll be doing and what to expect.

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## edKENdance

> Thanks very much for the insight everyone. Patients who come in having happily worn poorly measured glasses is always a bit tricky, but I agree with Boldt in terms of explaining exactly what you'll be doing and what to expect.


Just curious as to what you say when you're telling your client what to expect.

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## TheRobotious

To be honest I've only had to deal with this in terms of PDs being off, rather than seg height. Either way, I can't tell them exactly what to expect, instead I tell them they've gotten used to the 'wrong' measurements, and so they might feel as though the new glasses are a bit off. They might feel a little eye strain if it's a huge error, or a high power, but that it should become more comfortable reasonably quickly. Often though, the error isn't a huge problem, or they come in saying they're old glasses have never really felt right, so this makes it easy to explain why.

I suppose it's a similar conversation as if they're switching to an aspheric for the first time, and have gotten used to the distorted edge look in standard spherical lenses. It might seem somewhat 'off' at first, but once they're used to it, there's no problems.

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## optilady1

There are some people (not a lot) who for whatever reason do better when the segs are the same, no matter what.  It may be psychosomatic, because they don't want to admit that they aren't perfect.  One thing that I've learned over the years is, you cannot argue that you know better to the patient.  If you explain the situation calmly and logically, 99% of patients will agree with you, but the 1% who don't, you going over it again and again will only give you a headache.

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## TheRobotious

> One thing that I've learned over the years is, you cannot argue that you know better to the patient.


I've a barely been in the business for one year, and would agree this is absolutely paramount.

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## EyeCare Rich

I'm confussed here!  Why tell the patient the measurements at all, YOU are the expert.  Fit the glasses accordingly.  The more time you spend telling them you did something different, the more problem you are going to encounter.  Just adjust the frame, measure, record the information and order the glasses.  If there is an issue as far as adaptation, deal with it at the dispense, but if they were previously fit equal, and you find an unequal measurement, and order accordingly, they should be more comfortable as the eyes will track through the progressive channel a little easier, assuming the PD is measured correctly as well.

I'm surprised WMMCDONALD hasn't jumped all over this thread, as this is what he is always ramped up on, and it is showing here.  

Good luck TheRobotious, and welcome to Optiboard, read and learn as much as you can here, but also go and seek more knowledge from either coarsework/school/CE, and find a couple good optics books.

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## TheRobotious

Thanks EyeCare Rich, so far I've been doing just that!

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## optilady1

> I'm confussed here!  Why tell the patient the measurements at all, YOU are the expert.  Fit the glasses accordingly.  The more time you spend telling them you did something different, the more problem you are going to encounter.  Just adjust the frame, measure, record the information and order the glasses.  If there is an issue as far as adaptation, deal with it at the dispense, but if they were previously fit equal, and you find an unequal measurement, and order accordingly, they should be more comfortable as the eyes will track through the progressive channel a little easier, assuming the PD is measured correctly as well.
> 
> I'm surprised WMMCDONALD hasn't jumped all over this thread, as this is what he is always ramped up on, and it is showing here.  
> 
> Good luck TheRobotious, and welcome to Optiboard, read and learn as much as you can here, but also go and seek more knowledge from either coarsework/school/CE, and find a couple good optics books.


If I have a new patient purchasing progressives, I dot the old ones up so I can see what they've been wearing.  If it looks relatively normal, I do the new pair my way with no discussion.  If things are really weird, like a seg that starts at their eyelids for a progressives, I'm gonna ask why.  I don't care what the rules say, some patients want what they want, and if they are happy with what they have and they can articulate to me why they've worn their glasses like this happily, I'm not arrogant enough to tell them how they need to wear something.  This happens like twice a year.  Everything I do is to prevent someone coming back with issues.

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## TheRobotious

I think both points are valid, depending on the circumstances. If a previous patient is returning and gets an updated Rx, and I can see from the file that the previous optician was too lazy to measure near PDs for a reading pair (and just went with distance PDs), I will rectify this for the new pair without consulting the patient, and have never had any issues with this kind of thing. I think this is what EyeCare Rich is getting at.

If however, a patient comes in, and has a pair of glasses that deviate from the 'norm' considerably (as in Optilady's example), then I think it is both fair and safe to bring it up with the patient. It's likely been made up differently for a specific reason, and changing this with no discussion is asking for a remake.

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## EyeCare Rich

That makes absolute sense optilady1.  Good points made in your post.  I don't disagree with asking why something is odd and addressing it accordingly.  That is what I am talking about when I say be the professional.  I just don't think there needs to be a discussion for a slight uneven measurement.  Just adjust fit and dispense.  

Just my 2 pennies.

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## MakeOptics

For progressives and SV (hi RX) unequal fittings heights are preferred, but for lined multifocals experience has show me that the segments should be placed symmetrical in the frame otherwise the client assumes short work.  In lined multifocals however I will specify the DRP so the compromise is kept in the segment only and if the difference is too great then I use a larger set like a 35mm to avoid reducing the visual field.  Of course I'm talking about horizontal placement in this scenario.  Vertically the same holds true but I use the higher of the two measure in a bifocal and the lower in trifgocals.  The reason for not splitting lined is the average person can handle up to a difference of 3 prism diopters so I rely on their eyes to work a little harder which they are more than likely used to if they have been in eyewear in the past.

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## rbaker

Hopefully everyone responding to this query has worn either a FT or a PAL lens.

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## DanLiv

One thing to consider if segs are significantly different (2+mm) and fitting variable corridor freeform lenses, you will produce different corridor lengths between the lenses, which doesn't sound good to me. 1mm may not make much difference (although the fitting heights might be right on the cusp of where the software differentiates, and for some designs with only a few different corridor length this could end up with a 2mm corridor difference), but at 2+mm you are certainly going to alter the design and each eye will reach full add with a different downward gaze angle. For theses few patient's that I have had to do 2+mm dissimilar segs I either swapped into a fixed corridor design, or since I edge just ordered them from the lab at equal segs (to force the software to calculate equal corridors) and then edged dissimilar myself.

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## Robert Martellaro

> One thing to consider if segs are significantly different (2+mm) and fitting variable corridor freeform lenses, you will produce different corridor lengths between the lenses, which doesn't sound good to me. 1mm may not make much difference (although the fitting heights might be right on the cusp of where the software differentiates, and for some designs with only a few different corridor length this could end up with a 2mm corridor difference), but at 2+mm you are certainly going to alter the design and each eye will reach full add with a different downward gaze angle. For theses few patient's that I have had to do 2+mm dissimilar segs I either swapped into a fixed corridor design, or since I edge just ordered them from the lab at equal segs (to force the software to calculate equal corridors) and then edged dissimilar myself.


More Optiboard gold! Another reason to avoid variable corridor length designs altogether.

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## DanLiv

> Another reason to avoid variable corridor length designs altogether.


Do you consider them problematic in general Robert? I have had a few failures with deep 25mm+ seg heights where the near vision angle was too low, solved by just swapping in a fixed corridor.

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## Paul Smith LDO

Not to play the role of the Devil's Advocate here but, would not the refracting Dr pick up on any vertical image discrepancies and notate said correction for any unwanted vertical imbalance in the written Rx.  I can think of few reasons for denoting separate/split seg heights for PAL wearers.

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## Robert Martellaro

> Do you consider them problematic in general Robert? I have had a few failures with deep 25mm+ seg heights where the near vision angle was too low, solved by just swapping in a fixed corridor.


You've cited two really good examples of when the variable fails to provide proper function. 




> Not to play the role of the Devil's Advocate here but, would not the refracting Dr pick up on any vertical image discrepancies and notate said correction for any unwanted vertical imbalance in the written Rx.


I've never seen it. You give the docs and techs too much credit. I filled a -19 D Rx yesterday from an OD without a vertex distance noted. That will be my first call this morning.

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## Paul Smith LDO

Robert, you mean to say that you have never seen an Rx with a correction for vertical image imbalance.  That is exactly what I am referring to.  If a patient is displaying noticeable orbital asymmetry would we not expect vertical visual displacement and a prismatic correction for said vertical displacement.  I am speaking in general, as there are always exceptions.

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## ak47

> One thing to consider if segs are significantly different (2+mm) and fitting variable corridor freeform lenses, you will produce different corridor lengths between the lenses, which doesn't sound good to me. 1mm may not make much difference (although the fitting heights might be right on the cusp of where the software differentiates, and for some designs with only a few different corridor length this could end up with a 2mm corridor difference), but at 2+mm you are certainly going to alter the design and each eye will reach full add with a different downward gaze angle. For theses few patient's that I have had to do 2+mm dissimilar segs I either swapped into a fixed corridor design, or since I edge just ordered them from the lab at equal segs (to force the software to calculate equal corridors) and then edged dissimilar myself.


You will be asking for trouble if you put someone like this in a variable corridor lens.

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## ak47

> What do you do for clients who could be wearing uneven segs but have been wearing aligned segs for years?


I would split the difference in a progressive.  If they could be 4 mm apart but have been previously wearing them at same height OU, I would go 2mm apart.  I would certainly not say anything. 

If they came back a year or two later for another set, I would consider increasing further.

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## Robert Martellaro

> Robert, you mean to say that you have never seen an Rx with a correction for vertical image imbalance.  That is exactly what I am referring to.  If a patient is displaying noticeable orbital asymmetry would we not expect vertical visual displacement and a prismatic correction for said vertical displacement.  I am speaking in general, as there are always exceptions.


Paul,

If you mean without eyeglasses, I'll have to leave that to the vision experts to discuss. But I suspect that it wouldn't be a concern unless there was an injury to the the orbital area causing the eyes to suddenly misalign. If you mean prescribed prism to counter induced VI from improper lens positioning, read on.

It's the optician's job to eliminate VI whenever possible. For example, Rx is +10 sph OU. The right pupil is 2mm higher than the left pupil, and the frame is pre-fit. If we do not align the right OC 2mm higher than the left OC, the wearer will experience 2∆ of VI. 

This is pretty close to our fusional reserves, and unless habitual, may present symptoms. I have never seen a prescriber introduce prism, in the this case, 2∆ BU OD, with the assumption that the fitter is unprepared to position the lens correctly.

Best regards,

Robert M.

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## Paul Smith LDO

> You will be asking for trouble if you put someone like this in a variable corridor lens.


That is exactly the issue to what Dan is addressing.

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## Paul Smith LDO

It's the optician's job to eliminate VI whenever possible. For example, Rx is +10 sph OU. The right pupil is 2mm higher than the left pupil, and the frame is pre-fit. If we do not align the right OC 2mm higher than the left OC, the wearer will experience 2∆ of VI. 

Robert, no discord with the above, as I agree that we are here to provide our patients with the best option to maximize their VA.  I just don't see the necessity of split seg hts. for most PAL wearers.  When fitting patients for eyewear I tend to align the frame to the wearers eyes as they relate to the superior portion of the eyewire and not always their brow.  If we have to justify a split seg ht then I use a fixed corridor PAL design, but these are more of an exception to the rule, at our practice.

Cheers

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## Robert Martellaro

> Robert, no discord with the above, as I agree that we are here to provide our patients with the best option to maximize their VA.  I just don't see the necessity of split seg hts. for most PAL wearers.  When fitting patients for eyewear I tend to align the frame to the wearers eyes as they relate to the superior portion of the eyewire and not always their brow.  If we have to justify a split seg ht then I use a fixed corridor PAL design, but these are more of an exception to the rule, at our practice.
> 
> Cheers


OK, my turn to play Devil's advocate. Do you use binocular or monocular IPDs? Sauce for the goose... :Smile:

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## Paul Smith LDO

> OK, my turn to play Devil's advocate. Do you use binocular or monocular IPDs? Sauce for the goose...


REALLY!!? ...whats an IPD, I use binoculars at the stadium, my side kick LDO uses a monocle, where are you going with this Robert, or should I call you Beelzebub.   I use a digital pupilometer to gain the corneal apex reflex for the individuals mono IPD, I also use a, Haag-Streit, distometer for Vertex measurements and to administer a slight static jolt.

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## Robert Martellaro

> REALLY!!? ...whats an IPD, I use binoculars at the stadium, my side kick LDO uses a monocle, where are you going with this Robert, or should I call you Beelzebub.   I use a digital pupilometer to gain the corneal apex reflex for the individuals mono IPD, I also use a, Haag-Streit, distometer for Vertex measurements and to administer a slight static jolt.


Not fallen, but my halo is plenty bent.

Let me rephrase- if the horizontal meridian deserves the red carpet (monocular measurements), why not the vertical meridian? Don't we want the eyes to track downwards through the corridor equally, encountering the same power at all angles of downgaze? Doesn't this also improve horizontal symmetry, which has been shown to improve binocular function with PALs? Do the advantages overcome the disadvantages, pretty much limited to extra time and effort, and a slight change in spatial perception and prismatic effects, which are temporary and not enduring?

Envision a 7x28 trifocal fit level with a horizontal line tangent to the bottommost part of the pupil. Assuming a differential of 2mm in the height of each eye, using equal seg heights, which eye should be fit 1mm or 2mm above or below the bottom of the pupil, and why might it be better to match the position of the segment line with the position of the eyes?

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## MakeOptics

> Hopefully everyone responding to this query has worn either a FT or a PAL lens.


I wore a +2.00 for about 2 years in a PAL to vet myself.  Even without actively needing a PAL, I know how to fit them and love to make sure patients get the best vision through their lenses.  PAL's are a different animal and since they are indistinguishable from a SV lens to the consumer differing seg heights and accurate fitting a good idea to improve the vision, but with lined multifocals COSMETICS must be taken into account, I have been burnt one to many times by trying to be accurate when the client was never experiencing issues due to the imbalance.  I find the KISS principle applies well to lined multifocals, often times I find that even wearers with high Rx's that should be wearing a slab have more issues when I give them a slab since they have spent most of their lives suppressing and the minute I clear the lens up they start to experience double vision.  As human beings we have ways of compensating for these things.  Doctors have enough knowledge and experience to know that they need to often cut the astigmatism or power changes in older adults to avoid non-adapt issues.  Opticians learn the same with experience.  Your post seemed very raw and vague so I thought I would offer a bit of depth into my earlier explanation.

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## Robert Martellaro

I don't believe that it's necessary that we wear the lens design before it can be fit and designed it properly. Education and training are necessary though. 




> but with lined multifocals COSMETICS must be taken into account, I have been burnt one to many times by trying to be accurate when the client was never experiencing issues due to the imbalance.


Consider a switch to PALs if cosmetics are the priority.

If the client is new to me, or the change from equal to dissimilar segment heights is new, I cover my backside by telling my clients why I'm making the change, leaving no doubt as to the appearance of the lenses at dispense. Insets should also be handled in the same fashion: equal, or unequal, as needed, aligned with the eyes, compromising only when the disparities are large- where undercorrecting may be necessary to keep the nasal edge of the segment from being significantly truncated by the bridge. Never, ever, overcorrect.

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## ml43

> Paul,
> 
> If you mean without eyeglasses, I'll have to leave that to the vision experts to discuss. But I suspect that it wouldn't be a concern unless there was an injury to the the orbital area causing the eyes to suddenly misalign. If you mean prescribed prism to counter induced VI from improper lens positioning, read on.
> 
> It's the optician's job to eliminate VI whenever possible. For example, Rx is +10 sph OU. The right pupil is 2mm higher than the left pupil, and the frame is pre-fit. If we do not align the right OC 2mm higher than the left OC, the wearer will experience 2∆ of VI. 
> 
> This is pretty close to our fusional reserves, and unless habitual, may present symptoms. I have never seen a prescriber introduce prism, in the this case, 2∆ BU OD, with the assumption that the fitter is unprepared to position the lens correctly.
> 
> Best regards,
> ...


Spot on, just because someone has asymmetric pupil heights, does not mean they have diplopia. 

Just like tilting your head doesn't cause diplopia.  


My biggest problem with non symmetrical seg/fitting heights is we are assuming that the accommodative/convergent  pupil heights are the same as they are at infinity, which is not always true.

also, if you use an outside lab, they can be off by 1mm per eye.  So if you are starting at a 2mm difference, the lab can cut it with a 4mm difference, and it technically will still pass final inspection.

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## Robert Martellaro

> My biggest problem with non symmetrical seg/fitting heights is we are assuming that the accommodative/convergent  pupil heights are the same as they are at infinity, which is not always true.


This has to be quite rare! Do we call this nonconcomitant vertical tropia? Regardless, they probably won't be wearing multifocals.




> also, if you use an outside lab, they can be off by 1mm per eye.  So if you are starting at a 2mm difference, the lab can cut it with a 4mm difference, and it technically will still pass final inspection.


The tolerance is ±1mm for each eye, but the difference in vertical fitting point height should be no more than 1mm. I'm getting zero difference for fitting heights when specified as such, and less than one degree off-axis segment line and reference marks from Walman, Oak Creek WI. 

http://www.opticampus.com/tools/ansi.php

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## ml43

> This has to be quite rare! Do we call this nonconcomitant vertical tropia? Regardless, they probably won't be wearing multifocals.
> 
> The tolerance is ±1mm for each eye, but the difference in vertical fitting point height should be no more than 1mm. I'm getting zero difference for fitting heights when specified as such, and less than one degree off-axis segment line and reference marks from Walman, Oak Creek WI. 
> 
> http://www.opticampus.com/tools/ansi.php


haha, I'm not sure I would call it rare.  Just not common among PAL wearers, or not easily identifiable given the type of equipment opticians use on a regular basis for PAL fitting.  

Maybe most closely related to convergence disorders. 


I actually had to look up the z80.1 tolerances, and not just the summary, to make sure I am remembering things correctly.

As I read it, Vertical imbalance/prism only applies to binocular seg/fitting heights.

"6.2.3.2  Progressive Addition Lenses 

The vertical location (or height) of the fitting point for each progressive addition lens shall be within ±1.0 mm of specification. In addition, the difference between fitting point heights for the mounted pair shall not exceed 1.0 mm of specification. Measurement shall be made using the method in 8.7. 
The horizontal fitting point location in progressive addition lenses shall be within ± 1.0 mm of the specified monocular interpupillary distance for that lens. Measurement shall be made using the method in 8.7. "


Not that I personally would pass a pal with segments being off 1mm in each direction.  
But it does not say 1mm sum of differences from each specified height(perhaps it should?).

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## Robert Martellaro

> the difference between fitting point heights for the mounted pair shall not exceed 1.0 mm of specification.


I can only read that only one way. Here are a few examples. If I'm off-base, please give me a heads-up before the Google bots write it in stone.

If we order a fitting height of 20mm OU, 21/21, 21/20, 20/19, 19/19, 19/20, and 20/21 are all within tolerance. 21/19, 19/21 exceeds the 1mm difference and should be rejected.

If we ordered unequal heights of OD 22mm and OS 20mm, 23/20, 22/19 and so on would pass, but 23/19, 21/21 and so on would fail.

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## ml43

after rereading your quote of my quote of 6.2.3.2

It could, and should be interpreted as sum of differences.

I understood your implications from the beginning.  

But I know and have worked with techs who would pass over half of your examples.
Simply because that quote is not present on most ANSI z80.1 spec summaries/tables.

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