rebuild only if the output is not older than input, e.g. if they have the same modification times (this may be the case if you git pull), do not recompile Rmd
rebuild only if the output is not older than input, e.g. if they have the same modification times (this may be the case if you git pull), do not recompile Rmd

yihui/yihui.name,yihui/yihui.name,yihui/yihui.name