old_contents: from django import forms
  from django.contrib import admin
  
  from nested_inlines.admin import NestedModelAdmin, NestedStackedInline, NestedTabularInline
  from ordered_model.admin import OrderedModelAdmin
  
  from . import models
  
  
  class ChecklistStepInline(NestedTabularInline):
      model = models.ChecklistStep
      extra = 1
      fk_name = 'checklist_phase'
  
  
  class ChecklistPhaseInline(NestedStackedInline):
      model = models.ChecklistPhase
      extra = 1
      fk_name = 'checklist'
      inlines = [ChecklistStepInline]
      exclude = ('slug',)
  
  
  class ChecklistAdmin(NestedModelAdmin):
      model = models.Checklist
      inlines = [ChecklistPhaseInline]
      exclude = ('first_phase_slug',)
  
  
  class ChecklistStepAdmin(OrderedModelAdmin):
      list_filter = ['checklist_phase']
      list_display = ('item', 'action', 'move_up_down_links')
  
  
  admin.site.register(models.Aircraft)
  admin.site.register(models.Checklist, ChecklistAdmin)
  admin.site.register(models.ChecklistStep, ChecklistStepAdmin)
  
new_contents: from django import forms
  from django.contrib import admin
  
  from nested_inlines.admin import NestedModelAdmin, NestedStackedInline, NestedTabularInline
  from ordered_model.admin import OrderedModelAdmin
  
  from . import models
  
  
  class ChecklistStepInline(NestedTabularInline):
      model = models.ChecklistStep
      extra = 1
      fk_name = 'checklist_phase'
  
  
  class ChecklistPhaseInline(NestedStackedInline):
      model = models.ChecklistPhase
      extra = 1
      fk_name = 'checklist'
      inlines = [ChecklistStepInline]
  
  
  class ChecklistAdmin(NestedModelAdmin):
      model = models.Checklist
      inlines = [ChecklistPhaseInline]
  
  
  class ChecklistStepAdmin(OrderedModelAdmin):
      list_filter = ['checklist_phase']
      list_display = ('item', 'action', 'move_up_down_links')
  
  
  admin.site.register(models.Aircraft)
  admin.site.register(models.Checklist, ChecklistAdmin)
  admin.site.register(models.ChecklistStep, ChecklistStepAdmin)
  
subject: Remove dead code from admin.py