no empty command for system()
yihui updated branch pkg/knitr Wed Oct  8 21:22:42 CDT 2014
yihui updated branch pkg/knitr Thu Oct  9 02:35:08 CDT 2014
yihui updated branch pkg/knitr Thu Oct  9 12:23:33 CDT 2014
yihui updated branch pkg/knitr Thu Oct  9 14:26:57 CDT 2014