Comandos de Salvamento rescuers Maria Martinez (L) and Ana Chichilla attend to a wounded homeless man in San Salvador, El Salvador July 16, 2016. The man was attacked with a machete by suspected gang members. In 2015, El Salvador registered a record 103 homicides per 100,000 habitants, making it one of the most dangerous countries in the world outside a war zone. But for many young people who have few chances to distance themselves from rivalries between so-called maras in their schools and neighbourhoods, a civil-society organisation called the Comandos de Salvamento, or Rescue Corps, has been a refuge. (Photo by Jose Cabezas/Reuters)

Jason Arnold, project manager from solarcentury walks between lines of solar panels erected at Weighbridge, Wheal Jane, Baldhu on July 7, 2011 near Truro, England. A ceremony was held today to mark the connection of the 1.4kw solar farm, which on a 6.2 acre plot is the first in the South West and biggest in the UK to date, using 5,680 panels. (Photo by Matt Cardy/Getty Images)

Britain Soccer Football, West Ham United vs Manchester United, Barclays Premier League, Old Trafford on May 10, 2016. Police play football with a young West Ham fan before the match. (Photo by Eddie Keogh/Reuters/Livepic)

The Verreauxs Sifaka, otherwise known as Dancing Sifaka, who was caught strutting its stuff in Madagascar, has definitely learnt a lesson or two from King Julian – from the 2005 film “Madagascar”. Raising both of its arms and lunging from side to side, this lemur definitely likes to move it, move it. (Photo by Shannon Wild/Caters News Agency)

A workman takes a siesta on a girder during the building of Radio City, the city of New York spread out below. (Photo by General Photographic Agency/Getty Images). Circa 1933

Egyptian bootblacks cleaning a European's shoes in Cairo. (Photo by Hulton Archive/Getty Images). Circa 1870

A member of the German wireless police picks up signals on the radio equipment he carries on his back, while his colleague takes notes. (Photo by General Photographic Agency/Getty Images). Circa 1925

A Mark IV tank, part of the parade at the Lord Mayor's Show in London. (Photo by Topical Press Agency/Getty Images). November 1917