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examples: each one writes the rule edge and nothing else #1726

Job	Run time
toolchain: musl + llvm	3m 28s
toolchain: gcc (cold self-host)	3m 29s
examples: the curriculum builds	3m 28s
integration: mcpp builds & runs xlings	3m 28s
build + unit tests (linux x86_64, self-host)	3m 30s
	17m 23s