Revert "Merge branch 'separate-files'"

This reverts commit 34a6aaff1d34d6fed88afa63984c3c2e8950e224, reversing
changes made to 322c51b9f5037e1e635856f329da3a033e95f74d.
Revert "Revert "Merge branch 'separate-files'""

This reverts commit 5ccde769cc774f5992f770af1396a0d9e50eeff5.
add preface text