anchor.diff:  
   RUN DEBIAN_FRONTEND=noninteractive R CMD INSTALL \
      openintro_1.4.tar.gz \
  -   assertthat_0.1.tar.gz \
  +   assertthat_0.2.0.tar.gz \
      R6_2.2.0.tar.gz \
      magrittr_1.5.tar.gz \
      lazyeval_0.2.0.tar.gz \
  
positive.code: # libraries Eric Green wanted
  RUN DEBIAN_FRONTEND=noninteractive wget \
     http://archive.linux.duke.edu/cran/src/contrib/lubridate_1.6.0.tar.gz \
     http://archive.linux.duke.edu/cran/src/contrib/assertthat_0.1.tar.gz \
     http://archive.linux.duke.edu/cran/src/contrib/lazyeval_0.2.0.tar.gz \
     http://archive.linux.duke.edu/cran/src/contrib/tibble_1.3.0.tar.gz \
     http://archive.linux.duke.edu/cran/src/contrib/ggplot2_2.2.1.tar.gz \
  
negative.code: 
  RUN rm \
     openintro_1.4.tar.gz \
     assertthat_0.2.0.tar.gz \
     R6_2.2.0.tar.gz \
     magrittr_1.5.tar.gz \
     lazyeval_0.2.0.tar.gz \
  