anchor.diff:  RUN mkdir -p /demo/mongodb
   
   # Add APT credentials for MongoDB.
  -RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
  -RUN echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart' \
  -         ' dist 10gen' | tee /etc/apt/sources.list.d/10gen.list
  +RUN sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
  +RUN echo "deb http://repo.mongodb.org/apt/ubuntu "$(lsb_release -sc)"/mongodb-org/3.0 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.0.list
   
   # Install Ubuntu packages for Azrael.
   RUN apt-get update && apt-get install -y \
  
positive.code: # Default command: force grid demo.
  CMD ["/usr/bin/python3", "demo_forcegrid.py", \
       "--noviewer", \
       "--numcubes", "4,4,1"]
  
negative.code:     libassimp-dev \
      libassimp3 \
      libboost-python-dev \
      mongodb-org \
      python3-matplotlib \
      python3-netifaces \
      python3-numpy \
  