== id-based equals/hashCode, id assigned on save ==
two unsaved entities equal?  true  (both ids null)
set size after adding two different unsaved entities = 1
add, then save(): set.contains(c) = false

== constant hashCode ==
add, then save(): set.contains(d) = true

== natural key, immutable ==
add, then save(): set.contains(e) = true
