all(?) things at_exit and otherwise

output.txt

18930: Parent
18933: Child A
18934: Child B
18933: END in child if only exit called
18933: at_exit in child if only exit called
18933: parent END (only one)
18933: parent at_exit (2 times)
18933: parent at_exit (2 times)
18930: parent: child died
18930: parent trap(0)
18930: parent END (only one)
18930: parent at_exit (2 times)
18930: parent at_exit (2 times)

script.rb

puts "#{Process.pid}: Parent"
2.times { at_exit { puts "#{Process.pid}: parent at_exit (2 times)" } }
2.times {  END { puts "#{Process.pid}: parent END (only one)" } }
trap(0) { puts "#{Process.pid}: parent trap(0)" }
trap('CLD') { puts "#{Process.pid}: parent: child died" }
fork {
  puts "#{Process.pid}: Child A"
  at_exit { puts "#{Process.pid}: at_exit in child if only exit called" }
  END { puts "#{Process.pid}: END in child if only exit called" }
  trap(0) { "#{Process.pid}: trap(0) in child - NEVER HAPPENS" }
  exit
}
fork {
  puts "#{Process.pid}: Child B"
  at_exit { puts "#{Process.pid}: at_exit - NEVER HAPPENS" }
  END { puts "#{Process.pid}: END in child - NEVER HAPPENS" }
  trap(0) { "#{Process.pid}: trap(0) in child - NEVER HAPPENS" }
  exit!
}
Process.wait