/usr/include/core/ is in libprocess-cpp-dev 3.0.1-0ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/core/posix/
/usr/include/core/posix/child_process.h
/usr/include/core/posix/exec.h
/usr/include/core/posix/exit.h
/usr/include/core/posix/fork.h
/usr/include/core/posix/linux/
/usr/include/core/posix/linux/proc/
/usr/include/core/posix/linux/proc/process/
/usr/include/core/posix/linux/proc/process/oom_adj.h
/usr/include/core/posix/linux/proc/process/oom_score.h
/usr/include/core/posix/linux/proc/process/oom_score_adj.h
/usr/include/core/posix/linux/proc/process/stat.h
/usr/include/core/posix/linux/proc/process/state.h
/usr/include/core/posix/process.h
/usr/include/core/posix/process_group.h
/usr/include/core/posix/signal.h
/usr/include/core/posix/signalable.h
/usr/include/core/posix/standard_stream.h
/usr/include/core/posix/this_process.h
/usr/include/core/posix/visibility.h
/usr/include/core/posix/wait.h
/usr/include/core/testing/
/usr/include/core/testing/cross_process_sync.h
/usr/include/core/testing/fork_and_run.h