On the flag of Angola, the symbol with a half circle shape is part of three symbols, collectivley chosen to relate to the flag of what former contry?

The three symbols from the Angolan flag include: a cog wheel, machete and a star. These represent the regional variation of worker's tools, and workers tools in similar combination were also seen on the flag of the Soviet Union. It's also a symbol of communist, which is the mian reason why both countries adopted this flag.