anchor.diff:  , documentationSupport ? false
     , doxygen ? null
     , graphviz ? null
  - # GUI event viewer support
  -, eventGUISupport ? false
  + # GUI debug viewer support
  +, debugGUISupport ? false
     , cairo ? null
     , glib ? null
     , gtk3 ? null
  
positive.code: assert documentationSupport ->
    doxygen != null
    && graphviz != null;
  assert eventGUISupport ->
    cairo != null
    && glib != null
    && gtk3 != null;
  
negative.code:     libwacom
      mtdev
      systemd_lib
    ] ++ optionals debugGUISupport [
      cairo
      glib
      gtk3
  