refactor.
Split 'common/' into commit 'f430166ed9b3250ac871f99fd4f41c40238a238b'

git-subtree-dir: common
git-subtree-mainline: 5f7102190ee266a4171f4c10bbd2a21701e5ce0f
git-subtree-split: f430166ed9b3250ac871f99fd4f41c40238a238b