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CIA covert listening devices

EASYCHAIR – also EASY CHAIR or EC – was the codename of a secret research program, initiated by the US Central Intelligence Agency (CIA) and carried out by the Dutch Radar Laboratory (NRP) in Noordwijk (Netherlands). The aim of the project was to develop covert listening devices (bugs) based on the principle of the Resonant Cavity Microphone — also known as The Great Seal Bug or The Thing — that had been found in 1952 in the study of the US ambassador's residence in Moscow (Russia), hidden inside a gifted wooden carving of the Great Seal of the United States.

Immediately after the discovery of The Thing, the FBI and NRL started a formal investigation into the operation of the device. In parallel – separate from the FBI – the CIA started its own research program — codenamed EASYCHAIR and carried out by the NRP — to see if this technology could be of use to the CIA. The initial program ran from 1954 to 1967, but was extended several times. The relationship between the CIA and the NRP lasted until the demise of the latter in 1994.

 EASYCHAIR history

EASYCHAIR equipment on this website
Easy Chair Mark I - Passive Element (PE)
Easy Chair Mark II - Carrier Pigeon (CP)
Easy Chair Mark III - Extended range
Easy Chair Mark IV - High-power
Easy Chair Mark V - Multi-channel
Wired Easy Chair - room bugging via analogue telelephone set
Project 'Rocking Chair' (RC) - room bugging via telephone line
Tester for Easy Chair (Mk II)
Resonent Cavity Microphone for 360 and 1100 MHz
Pied Piper (PP) research project
The initial EASYCHAIR research program lasted from 1954 to approx. 1967. After that, the secret research continued under the same name, but the actual devices were no longer known as EASYCHAIR (EC) devices. Instead they were identified by CIA-provided designators, like SRT-107 and SRR-90. Nevertheless, the later devices produced by the NRP are often named EASYCHAIR to discriminate them from CIA bugs made by other suppliers. To avoid confusion, we will call the later devices post-EASYCHAIR throughout this website.
Post-EASYCHAIR equipment on this website
290 MHz bug with TP audio masking
350 MHz bug with RP audio masking
Rectangular version of SRT-56
250 Mhz ug with DSBSC audio masking
Low-power version of the SRT-91
Miniature 350 MHz transmitter (bug) with Dirty Pulse (DP) audio masking
1500 MHz covert transmitter with noise-based speech masking
Minature 290 MHz transmitter (bug) with SC audio masking
Minature 70 MHz switch-receiver
Subminiature transmitter developed as part of the Super Pulse project
SRR-52 listening post receiver
SRR-56 listening post receiver
Improved modular surveillance receiver (vertical model)
Improved modular surveillance receiver (horizontal model)
Modular surveillance receiver for RP and DP masked bugs
SRR-145 down-converter
SRR-153 surveillance receiver
QRT-153 activation transmitter
High-performance covert antenna
Directive listening post antenna for 350 MHz
1500 MHz antenna for Listening Post (1500 MHz)
Fixed 1500 MHz antenna for target (bug)
Directional 1500 MHz antenna for target (bug)
Measurement and test devices
Tester for Easy Chair (Mk II)
Path loss survey system
Path loss survey system (wideband)
Path loss survey system (narrowband)
Universal Radio Transmitter (part of URS-1) 312-320 MHz
Universal Radio Transmitter (part of URS-4) 169 MHz
Universal Radio Receiver (part of URS-1) 312-320 MHz
Universal Radio Receiver (part of URS-3) 1380 MHz
Single-ended VSWR meter for testing antenna matching
Inline VSWR meter
UVK-153 transmitter tester
Complete surveillance systems
SRS-53 surveillance radio system
SRT-56 transmitter consisting of RF unit, video coder and PSU
SRR-91 surveillance receiver
SRT-153 and QRR-153 in front of an SRR-153
On 4 August 1945, the Vladimir Lenin All-Union Pioneer Organisation presented a hand-carved replica of the Great Seal of the United States to US Ambassador Averell Harriman, as a gesture of friendship to the USSR's World War II (WWII) ally. Unknown to Harriman, the carving contained an hitherto unknown type of listening device, that became known as a resonant cavity microphone. The carving was hung in the Ambassador's study and provided the Soviets with the best possible intelligence for nearly seven years, before it was discovered during a staged sweep operation.

Immediately after the discovery of the device in September 1952, it was handed over to the FBI [1], where it was thoroughly investigated at their Technical Laboratory. After the first preliminary findings on 23 September, a full technical report was issued by the FBI on 1 December 1952 [2].

During the FBI investigation, the progress was constantly reported to a special commission, as directed by the President, consisting of the IIC, the ICIS and the CIA. Based on this information, the CIA built a number of functional replicas of the Thing for internal and external research [8].

It is often thought that the Americans had no idea how the device worked and that they turned in desperation to the UK for help, where part-time MI5 consultant Peter Wright solved the problem for them in about 10 weeks [3]. It is probably correct that Wright investigated the device and was able to build a British replica of it — codenamed SATYR — but he was certainly not the only one to be involved. Unknown to Wright, and probably also to MI5, the CIA started a secret research project under the codename EASYCHAIR. In his book Molehunt [4], David Wise writes about this:

The effort to copy the Soviet bug that had been discovered inside the Great Seal was given the code name EASY CHAIR by the CIA. The actual research was being performed in a laboratory in the Netherlands in two supersecret projects code-named MARK 2 and MARK 3.
A single paragraph in a single book referring to an unnamed Dutch laboratory was for many years the only clue that the Netherlands had been involved in the development of an equivalent of the Russian bug. In September 2015, in the article Operatie Leunstoel (Operation Easy Chair), the Dutch online magazine De Correspondent revealed that it was the Dutch Radar Laboratory [5].

 More about The Great Seal Bug

In late 1952, the CIA contacted the Dutch Internal Security Agency (BVD, now AIVD), in the hope that they could appoint engineers and scientists to help with their investigation. It was thought that the multinational Philips corporation might be able to help. On 28 October 1952, little more than one month after the discovery of the Thing, BVD officer Cees wrote a letter to Theo Tromp, member of the Philips Board of Directors, in which he reveals an unknown device (translated):

From the American side we've been confidentially informed that microphones with a diameter similar to a Rijksdaalder have been found at various American diplomatic facilities behind the Iron Curtain. These microphones contain a battery and are said to have a range of 10 km. [...]
They were found in easy chairs etc.

Do you think it possible that such devices exist and, if they do, what their lifespan would be? The Americans are rather upset about this, as the devices cannot be discovered in the usual manner. They are completely made of plastic. [...]
During WWII, Tromp had briefly been Minister of Traffic but also was an active member of the Dutch resistance group Harry. He also had frequent contacts with two major resistance groups: Ordedienst en Raad van Verzet (RVV) [10]. It is likely that he was trusted by the director of the BVD, Louis Einthoven, who who had recruted much of his personnel from the former resistance.

It is unknown who Tromps BVD contact Cees was, but it is possible, if not likely, that this was Cees van den Heuvel, the head of the Training and Research section of the BVD, who operated from his office at the Van Stolkweg in Den Haag. During the war, van den Heuvel was a member of the resistance group Albrecht; one of the few groups that was never captured by the Germans [11]. It is likely that the two men knew each other from their wartime underground work.

There are several issues with the above quoted letter [12]. No battery was used with this type of listening device, and it is unlikely that it was entirely made of plastic. Furthermore, a range of 10 km seems unrealistic, even by today's standards. It is possible that Cees' contact at the CIA was not technically knowledgeable or had (deliberately) been misinformed. What is remarkable though, is that these devices had reportedly been found in easy chairs (fauteuils), and that it had the diameter of a Rijksdaalder — a Dutch NLG 2.50 coin with a diameter of 33 or 38 mm.

 Read the original letter (in Dutch)

Apparently Tromp had the same reservations about the letter. In his answer the following day, he challenged the range of 10 km and rightfully remarked that a battery powered device would last hours instead of days, especially with the batteries that were available at the time [13]. In reply to the claim that the device was entirely made of plastic, he suggested the following:

The message that these devices exist and the addition that they appear to be completely made of plastic, could indicate that so-called Germanium triodes are used, also known as 'transistors'.
The transistor had been invented by Bell Labs in 1947, but by 1952 its development was still in its infacy and was not widely used in equipment. Tromp suggested that it might even be possible that the Russians had developed their own version of the transistor, and that they had used it in this novel device. Unfortunately, all of this was based on false and incomplete information.

 Read the full reply from Tromp

So far, the above is the only evidence that the BVD spoke with Philips about the discovery of the device. It is likely that in the following year, Philips Natuurkundig Laboratorium (later: Philips Research) was approached, but it turned out to be a dead end. Apparently, Philips was not interested in a follow-up, as it declined a request from BVD director van Einthoven to further research the secret technology. It is possible that Philips was too busy rebuilding their business in post-war Netherlands, or that they were only interested in high-volume consumer products.

Dutch Radar Laboratory (NRP)
Nederlands Radar Proefstation

After Philips had declined, the director of the BVD — Louis Einthoven [6] 1 — decided to consult Joop van Dijk — another wartime resistance friend. Van Dijk had just established the Dutch Radar Laboratory (NRP) in Noordwijk (Netherlands) in an attempt to bring the Netherlands up to speed with the latest developments in radar technology, with help from people like Watson-Watt [14].

The NRP was located right at the Dutch coast, and Noordwijk was conveniently close to Den Haag (The Hague) were both the BVD and the American embassy were based. Specialised in radar and UHF/SHF radio technology, the NRP appreared to be the ideal partner to secretly research the resonant cavity microphone.

During the course of 1954 2 the NRP was visited by people from the CIA, who told them that a mysterious bug had been found in an easy chair, hence the project name EASYCHAIR. The visit resulted in a research contract between the NRP and the CIA that would last for several decades.

At the time, the NRP was housed in a large villa, named Wave Guide, right at the Dutch coast in Noordwijk. It is shown in the image on the right and was the NRP headquarters for many years.

Involved in the EASYCHAIR research were NRP engineers Jan-Albert Bijvoet, Gerhard Prins and owner Joop van Dijk. Bijvoet would later be replaced by Thijs Hoekstra and At Admiraal. In the early years, the work was carried out in the evenings, after normal office hours when the rest of the workforce had left, but that changed after the first successful product was delivered in 1956.

The image on the right shows an Easy Chair Mark I, that consisted of two modules. It did not need a battery, as it was powered by a strong RF signal beamed at it from a nearby listening post.

In the following years, many EASYCHAIR (EC) devices were built for the CIA and also for the Dutch Internal Security Service (BVD), although the latter had to obtain the CIA's permission for each operation that involved an EC device. The NRP was even involved in planting some of the listening devices, such as the bugging of the Russian and Chinese embassies in The Hague.

Despite the close relationship with the CIA, it wasn't until 1960 that the NRP learned that the original resonant cavity mircophone was not found an an easy chair, but inside a wooden carving of The Greal Seal of the United States, that had been received as a gift from the Russians in 1945.

The cooperation between the NRP and the CIA that started in 1954, lasted until 1994, during which time many transmitters, receivers, bugs and test devices were developed and supplied to the CIA. According to Prins in [7], the CIA orders accounted for half the company's turnover (sometimes even more) for many years. This did not go unnoticed to the rest of the personnel of NRP and CHL, 3 who believed that the NRP just made test equipment for the US Military [9].

According to some people, the Dutch Intelligence Agency (BVD) was only partially aware of the level of cooperation between the NRP and the CIA. Much of the work for the CIA happened outside the view of the BVD. That said, new NRP personnel had to be screened by the BVD for many years, which implies that the BVD was not completely unaware. Besides, the NRP had an armed guard on site and were under surveillance of the Dutch Army during the night.

 More about the NRP

  1. During his wartime resistance work, Louis Einthoven was known as The Colonel.
  2. In a letter to his heirs [7], Gerhard Prins states that the first contact with the CIA was in 1954, but in the article in De Correspondent [5], the authors have changed this to 1952. Based on documents that are in our possession, we believe the former (1954) to be correct.
  3. At the time, the Christiaan Huygens Laboratory (CHL) was a subsidary of the NRP, located in an adjacent building. Today, CHL is an independant company.

CIA research on EASYCHAIR
The first contact between the CIA and the NRP dates back to 1954 (see above), but before the work on EASYCHAIR could start, the company, its directors and all personnel involved had to be screened by the BVD at the request of the CIA. Once approved, the work could commence.

Shortly after The Thing had been discovered in Moscow, the CIA was briefed on the matter by the Federal Bureau of Investigation (FBI). During the investigation, the CIA was kept up to speed via a special IIC-ICIS-CIA commission (SC), as directed by the US President (POTUS).

Based on this information, the CIA started its own research project with the aim to develop listening devices based on the same principle. The program was codenamed EASYCHAIR [4].

For internal and external research, the CIA had built a number of resonant cavity micro­phones that were very similar to The Thing, albeit with limited success. They were accompanied by a technical discription of their operation [8]. A cross section of the device is shown on the right.

It is doubtful whether the CIA had a good and full understanding of the device's operation, as their test results do not reflect the experiences of the FBI, the NRL and the SCEL, all of which built functional replicas in 1952 and reported that the device had excellent audio sensitivity.

Furthermore, the CIA device worked at a much lower frequency (1002 MHz) than the original Russian one (1700 MHz). Nevertheless they did some serious research, and saw room for future improvements. They gave the device a calibrated adjustable tuning post and replaced the nickel membrane by a mylar 1 one, upon which a thin aluminium layer had been evaporated [8].

Interestingly, the CIA has specified the antenna length in the drawing above at λ/2, whereas according to the preliminary findings of the FBI it was 1½λ long. This clearly indicates that the CIA had diverted from the original design and from the later replicas built by the FBI and the SCEL. As part of the project, a CIA replica was provided to the NRP in July 1955, along with the report [8].

 Read the complete uncensored CIA report

  1. Mylar, or BoPET, is a very strong and stable thin polyester film.   Wikipedia

NRP research on EASYCHAIR
Based on the information provided by the CIA [8], the NRP was unable to create a reliable replica of the Thing in 1955. Instead they investigated other possibilities. After initial experiments with 10 GHz (3 cm) signals had failed due to the high damping of the walls of a building, an operating frequency of 375 MHz was chosen. With just 40W of RF power, it appeared to be possible to use the bug in any room of the three-story building. It marked the start of a successful relationship with the CIA and the development of a range of covert listening devices and matching receivers.

  1. EC Mk I
    Passive Element (PE)
  2. EC Mk II
    Carrier Pigeon (bi-directional communication)
  3. EC Mk III
    Extended range and subcarrier modulation
  4. EC Mk IV
  5. EC Mk IV
    Multi-channel subcarrier
Easy Chair Mark I
In the early days, the engineers at the NRP had to work on EASYCHAIR in their own time, after office hours. Their first success came in late 1955. Using the diode and transistor technology that had just become available, they were able to build their first Passive Element 1 , or PE. A strong UHF signal, directed at the PE, was first rectified and then used to feed a 3-stage microphone amplifier, the load of which caused the reflected signal to be Amplitude Modulated (AM).

Block diagram of the Easy Chair Mark 1

In October 1956, Joop van Dijk and Gerhard Prins of the NRP travelled to the US, to demonstrate their solution to the Contracting Group of the CIA. The trip payed off, as it resulted in an order for 6 portable units that were identified as Easy Chair Mark I, or EC Mk I. It was the start of a successful relationship with the CIA that would last for more than three decades.

 More about Easy Chair Mark I

  1. Stricktly speaking, the device no longer was a Passive Element in the sense of the Thing, as active components (transistors) were involved. However, as it didn't need its own local power source, they kept referring to it as a Passive Element, or PE.

Later work
From this point onwards, the work was carried out during normal office hours in a new secured laboratory, with the evenings only needed for field tests. After a visit from CIA head Allan Dulles 1 in 1957, the green light was given for the development of the EC Mk II, III, IV and finally EC Mk V.

Easy Chair Mark V modulated the sound from the microphone onto a frequency-modulated (FM) subcarrier, allowing multiple bugs in the same target area to be received simultaneously at the listening post. They were activated by a 500W transmitter in combination with a 14dB corner reflector antenna, resulting in an ERP of 10kW.

In 1965, about ten years after starting to work for the CIA, the NRP had managed construct a reliably working replica of The Thing, and presented their Pulsed Cavity Microphone. One model for 360 MHz and one for 1100 MHz.

Unfortunately, their efforts never got beyond the prototype stage. In the preceeding years, both the Americans and the Russians had accused each other of 'bombarding' the other party with excessively strong radio signals (RF). In some cases this had reportedly led to health issues among embassy personnel. As a result, the CIA was no longer interested in this type of device.

Although the early (passive) EASYCHAIR devices worked very well, they had a number of serious limitations, such as a short operational range, the rather large listening post that was needed, and the very strong RF activation beam that sooner or later would be noticed by the target.

The NRP therefore started its own research in the field of Active Target Elements (ATE) which were powered by batteries, the mains network or a telephone line. It was important to keep the power consumption to a minimum, and have the ability to turn the device ON and OFF remotely.

Furthermore, it was recognised that any new devices had to be able to mask the audio modula­tion, so that they could not be picked up with a standard surveillance receiver [7]. The research resulted in a range of useful and innovative new techniques that were interesting enough for the CIA to keep doing business with the NRP for several decades. Due to the secret nature of the work, it was not possible for the NRP engineers to apply for patents 2 at the time.

  1. The presence of CIA head Allan Dulles at the demonstration in Wassenaar (Netherlands) in October 1957, is only mentioned in a letter in the estate of Gerhard Prins [7] and is not confirmed by any other public historical document. However, as he (Prins) and company director Joop van Dijk later both received a personally signed copy of Dulles' book The Craft of Intelligence, this might actually have been the case.
  2. During our research into the history of the NRP, we have found many creative and innovative designs, that have only recently been patented by other parties. The actual inventions however, were done at the NRP several decades earlier, but could not be published and/or patented at the time.

Known usage
Russian Embassy in The Hague
In 1958, in a joint operation of the CIA, the BVD and the NRP, the Russian Embassy in The Hague (Netherlands) was successfully bugged for more than 6 months with an Easy Chair III device.

The ambassador had just ordered new furniture for his office and the BVD managed to hide an Easy Chair III in one of the legs of a desk. It was activated by means of a powerful transmitter located at an attic about 250 metres away [7].

 Read the full story


Russian Embassy in Washington
In 1987, the American government accused the Soviets of bugging their embassy in Moscow. The Russians replied with a press conference on 10 April 1987, saying that the American claims were unfounded, and that instead they (the Russians) were bugged by the Americans.

At the press conference they showed a variety of captured bugs, one of which (in the red circle) can be identified as a Easy Chair SRT-56 device that had probably been planted in 1971/1972.

 Read the full analysis


Known Easy Chair products
Below is a list of products that are known to have been developed by the NRP for the CIA under the EASYCHAIR contract. The NRP was by no means the only developer of covert listening devices for the CIA however. For a more complete list of devices, please check our special CIA page.

Early Easy Chair products
Complete surveillance systems
Measuring equipment
Audio masking techniques

Project names
EC   Easy Chair
Codename for the CIA's early research into resonant cavity microphones, between 1952 and 1955. Also the global name for the long-term research contract between the CIA and the NRP. In addition, some products made by the NRP carried the name Easy Chair or EC.
CP   Carrier Pigeon
Method of carrying speech modulation in the activation beam of an Easy Chair installation, effectively creating a two-way system. Carrier Pigeon, or CP, was used in the EC II system.
DP   Dirty Pulse
Audio masking scheme, based on Pulse Position Modulation, in which random noise is added to the rising edge of each pulse.  More
JJ   ?
Details currently unknown.
MB   Music Box
Slot antenna for Easy Chair, developed by CIA (November 1956).
O   Omelette
Development of an alternative voice scrambler system, that defeats existing classical decrambling methods. September 1956.
PP   Pied Piper
Research project for the development of a remote controlled radio bug that used the TP audio masking scheme. The project eventually led to the development of the SRT-40.  More
RC   Rocking Chair
System for bugging a room by connecting a Passive Element(PE) to an analogue telephone line and activating it with a WEC device connected at a tapping point.  More
RP   Rejected Pulse
Audio masking scheme, based on Pulse Position Modulation, in which noise is used to randomly reject some of the pulses. This is arguably one of the most effective analogue audio masking schemes ever developed.  More
SP   Super Pulse
Research project for an alternative audio masking scheme based on Pulse Position Modulation — later merged with Dirty Pulse (DP) — and miniature transmitter.  More
TP   Triple Pulse
Audio masking scheme, based on Pulse Position Modulation, involving the use of three consecutive pulses. This masking scheme was also used by Pied Piper (SRT-40).  More
AM   Amplitude Modulation
ATE   Active Target Element
Active bugging device that has its own power source, can be controlled remotely and has full masking of its audio signal. Also known as TE.
BVD   Binnenlandse Veiligheidsdienst
Dutch internal security service, later renamed to AIVD.  More
CIA   Central Intelligence Agency
United Status foreign intelligence agency.  More
CIA codename for the super-secret project to develop covert listening devices based on the principle of the Russian resonant cavity microphone, also known as The Thing. The name EASY CHAIR is also written as EASYCHAIR.
ERP   Effectively Radiated Power
FBI   Federal Bureau of Investigation
United States federal law enforcement agency, tasked with the investigation of federal (cyber) crime, corruption, terrorism and counterintelligence.  More
FM   Frequency Modulation
NRL   Naval Research Laboratory
Technical research laboratory of the US Navy.
NRP   Nederlands Radar Proefstation
Dutch Radar Laboratory, at the time located in Noordwijk (Netherlands).  More
MI5   Military Intelligence 5
British internal intelligence agency.  More
PE   Passive Element
Covert listening device (bug), that does not require a local power source. Instead it is powered by the energy from a strong RF signal beamed at it from a nearby location. Examples are the russian resonant cavity microphone and the CIA's Easy Chair devices.
Potus   President of The United States
Generic name for the (current) president of the United Status. When male, his female partner is usually known as First Lady of the United States (FLOTUS).
SC   Special Commission
Special intelligence commission established by the US President, consisting of the IIC, ICIS and CIA, to share research information about the Russian resonant cavity microphone (The Thing).
SCEL   Signal Corps Engineering Laboratories
Technical engineering laboratory of the US Army Signal Corps, based at Fort Monmouth (New Jersey, USA).
SHF   Super High Frequency
Common expression for frequencies of 3000 MHz and higher.
TE   Target Element
Bugging device that is powered by a local battery or another local power source. Also known as Active Target Element (ATE).
UHF   Ultra High Frequency
Common expression for frequencies in the range 300 - 3000 MHz.
WWII   World War II
Also known as World War 2, or the Second World War.

  1. I.W. Conrad to Mr. Harbo, Results of laboratory examination
    Internal FBI memorandum. 23 September 1952. 1

  2. Director FBI to POTUS, Release of FBI report on Russian microphone
    Official letter, via Matt Connelly, to the US President. 4 December 1952. 1

  3. Peter Wright, Spycatcher
    ISBN 0-440-29504-1. 1987-1988.

  4. David Wise, Molehunt
    ISBN 978-0394585147. 10 March 1992.

  5. Maurits Martijn & Cees Wiebes, Operation Easy Chair
    De Correspondent. 24 September 2015.

  6. Wikipedia, Louis Einthoven
    Dutch. Retrieved October 2015.

  7. Gerhard Prins, Letter to his heirs
    Date unknown, but probably written shortly before his death in April 1993.
    Vertrouwelijk (confidential). Published by [5].

  8. CIA Contracting Group, Report on Research on EASYCHAIR
    14 July 1955. Classification status unknown. Not marked as secret.

  9. Nederlands Radar Proefstation 40 Jaar / 1947-1987
    July 1987. Chapter 'Amerikaanse zaken' (American affairs).

  10. Wikipedia, Theo Tromp
    Visited 21 May 2023.   Wikipedia (Netherlands)

  11. Philip van de Poel, Succes en falen van de Nederlandse inlichtingendiensten
    Historisch Nieuwblad, 10 October 2012.

  12. Cees
    , personal letter to Ir. Theo P. Tromp (Philips)

    The Hague (Netherlands), 28 October 1952. In Dutch language. 2

  13. Theo Tromp (Philips), letter to Cees
    (in Dutch language)

    Eindhoven (Netherlands), 29 October 1952. GEHEIM (secret). 2

  14. Wikipedia, Robert Watson-Watt
    Visited 21 May 2023.
  1. Released by the FBI under the Freedom of Information Act (FIOA) on 11 May 2014 (FOI/PA# 1173422-1).
  2. Obtained from the Dutch National Archives (NA) by Maarten Oberman in April 2023.

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Crypto Museum. Created: Tuesday 12 January 2016. Last changed: Sunday, 26 May 2024 - 10:52 CET.
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