Formal Specification using Z (Grassroots, 4)

In 2002, World of Books Group was founded on an ethos to do good, protect the planet and support charities by enabling more goods to be reused. Since then, we've grown into to a global company pioneering the circular economy. Today, we drive the circular economy through three re-commerce brands: - Wob: Through Wob, we sell. We provide affordable, preloved books and media to customers all over the world. A book leaves our collection of over seven million titles and begins a new chapter every two seconds, enabling more goods to be reused. - Ziffit: Through Ziffit, we buy. We give people around the world the opportunity to contribute to the circular economy, earn money and protect the planet, by trading their unwanted books and media. - Shopiago: Through Shopiago, we help others. By sharing the technology that has grown World of Books Group into the business it is today, we’re helping charities increase revenue and reduce waste through re-commerce.…