the other problem

• Let’s say I want to try something with an old version of Professor

  • ... which depends on Rivet
    • ... which depends on root
      • ... which ultimately depends on gcc
    • ... which depends on TeXLive
      • ... which depends on perl
  • ... which depends on matplotlib
    • ... which depends on python
      • ... which depends on gcc

• all of these might depend on versions you don't have on your system

• recipe for disaster if you tried it anyway

• all just for a quick experiment