Only later is it discovered that wrapped within it is a fingernail-sized piece of circuit board with a tell-tale identifying mark - the number one.

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Shot predominantly in black and white, with spot colour for a Jewish child's red coat the film's unique identifying mark it recalls the tradition of post-war Italian neo-realism and the even older photographs of Roman Vishniac of Jewish life in Poland before the Nazis.

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The Met's Supt Mark Goldby, who is leading Prevent, said it was not about spying on people, just identifying those who were at risk.

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