When I run synaptic, there are certain packages which synaptic treats as "essential". For example, if I select bash for removal, I will be warned that bash is an "essential package". If I then press the apply button I get a list of packages that are going to be acted on. One of those lists is the list of essential packages that I am about to remove.
Using the apt toolset, how can I vgenerate a list of all the packages that synaptic considers essential?