Revert "change direction up to down" (#452)
* Revert "Update preface.md" This reverts commit e42035ec. * Revert "change direction up to down (#451)" This reverts commit 534b782d.
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* Revert "Update preface.md" This reverts commit e42035ec. * Revert "change direction up to down (#451)" This reverts commit 534b782d.